mxc-mmc: sdhc host driver for MX2 and MX3 proccessor

This is a port of Linux driver for SDHC host controller hardware
found on Freescale's MX2 and MX3 processors. Uses new generic MMC
framework (CONFIG_GENERIC_MMC) and it looks like there are some
problems with a framework (at least on LE cpus). Some of these
problems are addressed in the following patches.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>