imx: imx6sx-sdb: Enable DM QSPI driver

To support DM QSPI driver
 - Add spi0 and spi1 alias for qspi1 and qspi2.
 - Add -u-boot.dtsi to modify n25q256a@0 and n25q256a@1 compatible string
   to "spi-flash" and add "num-cs" property.
 - Enable DM SPI/QSPI relavent configurations
 - Remove iomux settings of qspi2 in board codes which is not needed
   for DM driver.
 - Add sf default settings. So running "sf probe" can detect the flash

Signed-off-by: Ye Li <ye.li@nxp.com>
diff --git a/configs/mx6sxsabresd_defconfig b/configs/mx6sxsabresd_defconfig
index dcac6f4..4c235c1 100644
--- a/configs/mx6sxsabresd_defconfig
+++ b/configs/mx6sxsabresd_defconfig
@@ -15,6 +15,7 @@
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y
+CONFIG_CMD_SF=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_PART=y
 CONFIG_CMD_PCI=y
@@ -37,8 +38,14 @@
 CONFIG_DM_I2C=y
 CONFIG_DM_MMC=y
 CONFIG_FSL_ESDHC=y
+CONFIG_SPI=y
+CONFIG_DM_SPI=y
+CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_BAR=y
+CONFIG_SPI_FLASH_STMICRO=y
+CONFIG_SPI_FLASH_SPANSION=y
+CONFIG_FSL_QSPI=y
 CONFIG_PHYLIB=y
 CONFIG_MII=y
 CONFIG_PCI=y