imx: imx6sx-sdb: Enable DM QSPI driver

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>