mmc: mxs-mmc: use devm_* helper to make cleanup simpler

Use devm_request_and_ioremap and devm_request_irq helpers to clean up
the code a little bit.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Chris Ball <cjb@laptop.org>