Create drivers/mmc subdirectory

In order to consolidate more of the various MMC drivers around the
tree, we must first have a common place to put them.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Acked-by: Jean-Chritophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>