mmc: mxs-mmc: Let device core handle pinctrl

Since commit ab78029 (drivers/pinctrl: grab default handles from
device core), we can rely on device core for handling pinctrl.

So remove devm_pinctrl_get_select_default() from the driver.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Chris Ball <cjb@laptop.org>