MLK-14775 mx7dsabresd: Disable USDHC2 in device tree

MLK-14775 mx7dsabresd: Disable USDHC2 in device tree

The USDHC2 is connecting to BT/WIFI chip on SDB board, this controller is enabled
in device tree as SDIO, but USDHC driver in u-boot will use it as SDHC. So totally
3 USDHC devices will be created, and cause run time MMC environments go wrong because
it only supports USDHC1 and USDHC3.

So disable the unused USDHC2 controller in u-boot device tree.

Signed-off-by: Ye Li <ye.li@nxp.com>