ENGR00175080 [MX6] MMC: kernel failed to init eMMC card, after boot from eMMC

ENGR00175080 [MX6] MMC: kernel failed to init eMMC card, after boot from eMMC

issue:
if uboot is loaded from eMMC, the eMMC memory will be configured to DDR mode.
on kernel startup, it will initialize the card at SDR mode, while the register
of USDHC is still configured to DDR enable mode. Therefore, the initialization
of eMMC memory will fail.

fix:
- clear MIX_CTRL on sdhc platform init code.
- clear vselect bit of VENDOR_SPEC on sdhc platform init code.

Signed-off-by: Ryan QIAN <b32804@freescale.com>