mips: Add basic MediaTek MT7620/88 support

This patch adds basic support for the MediaTek MT7620/88 SoCs. Parts of
the code is copied from the MediaTek GitHub repository:

https://github.com/MediaTek-Labs/linkit-smart-uboot.git

The mt7628a.dtsi file is imported from Linux v4.17.

Support for the LinkIt Smart 7688 module and the Gardena Smart Gateway
both based on the MT7688 will be added in further patches.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 30fb9bf..10b55c9 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -69,6 +69,21 @@
 	select SYSRESET
 	imply CMD_DM
 
+config ARCH_MT7620
+	bool "Support MT7620/7688 SoCs"
+	imply CMD_DM
+	select DISPLAY_CPUINFO
+	select DM
+	select DM_SERIAL
+	imply DM_SPI
+	imply DM_SPI_FLASH
+	select MIPS_TUNE_24KC
+	select OF_CONTROL
+	select ROM_EXCEPTION_VECTORS
+	select SUPPORTS_CPU_MIPS32_R1
+	select SUPPORTS_CPU_MIPS32_R2
+	select SUPPORTS_LITTLE_ENDIAN
+
 config MACH_PIC32
 	bool "Support Microchip PIC32"
 	select DM
@@ -121,6 +136,7 @@
 source "arch/mips/mach-ath79/Kconfig"
 source "arch/mips/mach-bmips/Kconfig"
 source "arch/mips/mach-pic32/Kconfig"
+source "arch/mips/mach-mt7620/Kconfig"
 
 if MIPS