mmc: Add a MX2/MX3 specific SDHC driver

This patch adds a MX2/MX3 specific SDHC driver. The hardware is basically
the same as in the MX1, but unlike the MX1 controller the MX2
controller just works as expected. Since the MX1 driver has more
workarounds for bugs than anything else I had no success with supporting
MX1 and MX2 in a sane way in one driver.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>