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

The gpio-mxs controller complies with basic_mmio_gpio library.  This
patch converts the driver to use the library.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>