mmc: mxcmmc: use slot-gpio API for write-protect detection

slot-gpio API suppors read-only detection when "wp-gpios"
property is present in the device tree mmc node. Use this
API for write-protect detection.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>