uboot-imx

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>