imxmmc: init-exit rework

Add __init __exit for appropriate probe and remove functions.
Conver to platform_driver_probe()

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>