MMC: Add 8-bit bus width support

[Jarkko Lavinen]

Hi Pierre

Here is a patch which adds 8-bit bus width suport to the mmc
driver common part.  

I've tested it with a test board with 8-bit eMMC device on Omap
3420 HSMMC controller. It speeds up DMA transfer from host to the
card and most of the time is spent in busy polling for the card
to become ready.

Regards
Jarkko Lavinen