ARM: mxs/mx28evk: add mmc device

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Tested-by: Marc Kleine-Budde <mkl@pengutronix.de>
[sha: updated to Uwes v4 version]
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>