imx: imx6sx-sabreauto: convert to use DM QSPI driver

To support DM QSPI driver:
 - Add -u-boot.dtsi to modify n25q256a@0 and n25q256a@1 compatible string
   to "spi-flash" and add "num-cs" property.
 - Enable DM SPI and DM SPI FLASH configurations
 - Remove iomux settings of qspi1 in board codes which is not needed
   for DM driver.

Signed-off-by: Ye Li <ye.li@nxp.com>
diff --git a/configs/mx6sxsabreauto_defconfig b/configs/mx6sxsabreauto_defconfig
index 359b9b6..0e15bd6 100644
--- a/configs/mx6sxsabreauto_defconfig
+++ b/configs/mx6sxsabreauto_defconfig
@@ -36,6 +36,8 @@
 CONFIG_FSL_ESDHC=y
 CONFIG_NAND=y
 CONFIG_NAND_MXS=y
+CONFIG_DM_SPI=y
+CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_BAR=y
 CONFIG_SPI_FLASH_STMICRO=y