mx6qsabresd: add usdhc2 and usdhc4 support

mx6qsabresd: add usdhc2 and usdhc4 support

The on-board number of available usdhc devices is something board
specific.  The patch moves CONFIG_SYS_FSL_USDHC_NUM out of
mx6qsabre_common.h and adds usdhc2 and usdhc4 support for mx6qsabresd
board.

To keep the default mmc device for environment same as before (usdhc3),
it moves CONFIG_SYS_MMC_ENV_DEV out of mx6qsabre_common.h and changes
it to 1 for mx6qsabresd.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>