ARM: mxs: dynamically allocate mmc device

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
[ukleinek: fix naming to include complete device name in functions]
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>