mmc: mxs-mmc: add mmc host driver for i.MX23/28

This adds the mmc host driver for Freescale MXS-based SoC i.MX23/28.
The driver calls into mxs-dma via generic dmaengine api for both pio
and data transfer.

Thanks Chris Ball for the indentation patch.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>