ARM: dts: imx6qdl-sabresd: Add USDHC4 support

USDHC4 is connected to a DDR MMC. Add support for it.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>