imx: mx6sxsabresd: Add board support for USDHC2 and USDHC3

Add full support for USDHC2, USDHC3, USDHC4 on mx6sx sabresd board.
The default boot socket is USDHC4, so the MMC environment device and
mmcdev variable are set to this device.

Signed-off-by: Ye.Li <B37916@freescale.com>
2 files changed