gpio/mxc: convert gpio-mxc to use basic_mmio_gpio library

The gpio-mxc controller complies with basic_mmio_gpio library.  The
patch convert the driver to use the library.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>