board: mediatek: add MT7988 reference boards

This patch adds general board files based on MT7988 SoCs.

MT7988 uses one mmc controller for booting from both SD and eMMC,
and the pins of mmc controller booting from SD are also shared with
one of spi controllers.
So two configs are need for these boot types:

1. mt7988_rfb_defconfig - SPI-NOR, SPI-NAND and eMMC
2. mt7988_sd_rfb_defconfig - SPI-NAND and SD

diff --git a/configs/mt7988_rfb_defconfig b/configs/mt7988_rfb_defconfig
new file mode 100644
index 0000000..dc97bb3
--- /dev/null
+++ b/configs/mt7988_rfb_defconfig
@@ -0,0 +1,83 @@
+CONFIG_ARM=y
+CONFIG_SYS_HAS_NONCACHED_MEMORY=y
+CONFIG_POSITION_INDEPENDENT=y
+CONFIG_ARCH_MEDIATEK=y
+CONFIG_TEXT_BASE=0x41e00000
+CONFIG_SYS_MALLOC_F_LEN=0x4000
+CONFIG_NR_DRAM_BANKS=1
+CONFIG_DEFAULT_DEVICE_TREE="mt7988-rfb"
+CONFIG_SYS_PROMPT="MT7988> "
+CONFIG_TARGET_MT7988=y
+CONFIG_DEBUG_UART_BASE=0x11000000
+CONFIG_DEBUG_UART_CLOCK=40000000
+CONFIG_SYS_LOAD_ADDR=0x46000000
+CONFIG_DEBUG_UART=y
+# CONFIG_AUTOBOOT is not set
+CONFIG_DEFAULT_FDT_FILE="mt7988-rfb"
+CONFIG_LOGLEVEL=7
+CONFIG_LOG=y
+CONFIG_SYS_CBSIZE=512
+CONFIG_SYS_PBSIZE=1049
+# CONFIG_BOOTM_NETBSD is not set
+# CONFIG_BOOTM_PLAN9 is not set
+# CONFIG_BOOTM_RTEMS is not set
+# CONFIG_BOOTM_VXWORKS is not set
+# CONFIG_CMD_ELF is not set
+CONFIG_CMD_CLK=y
+CONFIG_CMD_DM=y
+CONFIG_CMD_GPIO=y
+CONFIG_CMD_PWM=y
+CONFIG_CMD_MMC=y
+CONFIG_CMD_MTD=y
+CONFIG_CMD_PING=y
+CONFIG_CMD_SMC=y
+CONFIG_DOS_PARTITION=y
+CONFIG_EFI_PARTITION=y
+CONFIG_PARTITION_TYPE_GUID=y
+CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
+CONFIG_NET_RANDOM_ETHADDR=y
+CONFIG_USE_IPADDR=y
+CONFIG_IPADDR="192.168.1.1"
+CONFIG_USE_NETMASK=y
+CONFIG_NETMASK="255.255.255.0"
+CONFIG_USE_SERVERIP=y
+CONFIG_SERVERIP="192.168.1.2"
+CONFIG_PROT_TCP=y
+CONFIG_REGMAP=y
+CONFIG_SYSCON=y
+CONFIG_CLK=y
+CONFIG_MMC_HS200_SUPPORT=y
+CONFIG_MMC_MTK=y
+CONFIG_MTD=y
+CONFIG_DM_MTD=y
+CONFIG_MTD_SPI_NAND=y
+CONFIG_DM_SPI_FLASH=y
+CONFIG_SPI_FLASH_SFDP_SUPPORT=y
+CONFIG_SPI_FLASH_EON=y
+CONFIG_SPI_FLASH_GIGADEVICE=y
+CONFIG_SPI_FLASH_ISSI=y
+CONFIG_SPI_FLASH_MACRONIX=y
+CONFIG_SPI_FLASH_SPANSION=y
+CONFIG_SPI_FLASH_STMICRO=y
+CONFIG_SPI_FLASH_WINBOND=y
+CONFIG_SPI_FLASH_XMC=y
+CONFIG_SPI_FLASH_XTX=y
+CONFIG_SPI_FLASH_MTD=y
+CONFIG_PHY_FIXED=y
+CONFIG_MEDIATEK_ETH=y
+CONFIG_PINCTRL=y
+CONFIG_PINCONF=y
+CONFIG_PINCTRL_MT7988=y
+CONFIG_POWER_DOMAIN=y
+CONFIG_MTK_POWER_DOMAIN=y
+CONFIG_DM_PWM=y
+CONFIG_PWM_MTK=y
+CONFIG_RAM=y
+CONFIG_DM_SERIAL=y
+CONFIG_MTK_SERIAL=y
+CONFIG_SPI=y
+CONFIG_DM_SPI=y
+CONFIG_MTK_SPIM=y
+CONFIG_LZO=y
+CONFIG_HEXDUMP=y
+# CONFIG_EFI_LOADER is not set
diff --git a/configs/mt7988_sd_rfb_defconfig b/configs/mt7988_sd_rfb_defconfig
new file mode 100644
index 0000000..421999d
--- /dev/null
+++ b/configs/mt7988_sd_rfb_defconfig
@@ -0,0 +1,71 @@
+CONFIG_ARM=y
+CONFIG_SYS_HAS_NONCACHED_MEMORY=y
+CONFIG_POSITION_INDEPENDENT=y
+CONFIG_ARCH_MEDIATEK=y
+CONFIG_TEXT_BASE=0x41e00000
+CONFIG_SYS_MALLOC_F_LEN=0x4000
+CONFIG_NR_DRAM_BANKS=1
+CONFIG_DEFAULT_DEVICE_TREE="mt7988-sd-rfb"
+CONFIG_SYS_PROMPT="MT7988> "
+CONFIG_TARGET_MT7988=y
+CONFIG_DEBUG_UART_BASE=0x11000000
+CONFIG_DEBUG_UART_CLOCK=40000000
+CONFIG_SYS_LOAD_ADDR=0x46000000
+CONFIG_DEBUG_UART=y
+# CONFIG_AUTOBOOT is not set
+CONFIG_DEFAULT_FDT_FILE="mt7988-sd-rfb"
+CONFIG_LOGLEVEL=7
+CONFIG_LOG=y
+CONFIG_SYS_CBSIZE=512
+CONFIG_SYS_PBSIZE=1049
+# CONFIG_BOOTM_NETBSD is not set
+# CONFIG_BOOTM_PLAN9 is not set
+# CONFIG_BOOTM_RTEMS is not set
+# CONFIG_BOOTM_VXWORKS is not set
+# CONFIG_CMD_ELF is not set
+CONFIG_CMD_CLK=y
+CONFIG_CMD_DM=y
+CONFIG_CMD_GPIO=y
+CONFIG_CMD_PWM=y
+CONFIG_CMD_MMC=y
+CONFIG_CMD_MTD=y
+CONFIG_CMD_PING=y
+CONFIG_CMD_SMC=y
+CONFIG_DOS_PARTITION=y
+CONFIG_EFI_PARTITION=y
+CONFIG_PARTITION_TYPE_GUID=y
+CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
+CONFIG_NET_RANDOM_ETHADDR=y
+CONFIG_USE_IPADDR=y
+CONFIG_IPADDR="192.168.1.1"
+CONFIG_USE_NETMASK=y
+CONFIG_NETMASK="255.255.255.0"
+CONFIG_USE_SERVERIP=y
+CONFIG_SERVERIP="192.168.1.2"
+CONFIG_PROT_TCP=y
+CONFIG_REGMAP=y
+CONFIG_SYSCON=y
+CONFIG_CLK=y
+CONFIG_MMC_HS200_SUPPORT=y
+CONFIG_MMC_MTK=y
+CONFIG_MTD=y
+CONFIG_DM_MTD=y
+CONFIG_MTD_SPI_NAND=y
+CONFIG_PHY_FIXED=y
+CONFIG_MEDIATEK_ETH=y
+CONFIG_PINCTRL=y
+CONFIG_PINCONF=y
+CONFIG_PINCTRL_MT7988=y
+CONFIG_POWER_DOMAIN=y
+CONFIG_MTK_POWER_DOMAIN=y
+CONFIG_DM_PWM=y
+CONFIG_PWM_MTK=y
+CONFIG_RAM=y
+CONFIG_DM_SERIAL=y
+CONFIG_MTK_SERIAL=y
+CONFIG_SPI=y
+CONFIG_DM_SPI=y
+CONFIG_MTK_SPIM=y
+CONFIG_LZO=y
+CONFIG_HEXDUMP=y
+# CONFIG_EFI_LOADER is not set