On iMX7D SabreSD board, the QSPI has pins conflict with EPDC (default).
To use QSPI, users have to rework the board (de-populate R388-R391, R396-R399
populate R392-R395, R299, R300). So we add new DTS file and new defconfig
dedicated for QSPI.

Other changes to support the DM QSPI:
 - Add QSPI node and alias spi0.
 - Modify spi4 (spi-gpio) node and add alias spi5 for it to avoid req
   conflict
 - Add EPDC node in imx7d.dtsi and disable it in imx7d-sdb-qspi.dts to
   align with kernel and also present the conflict.
 - Add -u-boot.dtsi to modify compatible string of mx25l51245g@0 to
   "spi-flash"
 - Remove iomux settings of qspi in board codes which is not needed
   for DM driver.
Signed-off-by: NYe Li <ye.li@nxp.com>