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>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index 4e2d612..531bfb6 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -101,6 +101,7 @@
 config GPIO_MXS
 	def_bool y
 	depends on ARCH_MXS
+	select GPIO_GENERIC
 
 config GPIO_PLAT_SAMSUNG
 	def_bool y