Convert CONFIG_MXC_GPIO to Kconfig

This converts the following to Kconfig:
   CONFIG_MXC_GPIO

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index b121979..34d4409 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -108,6 +108,11 @@
 	  - APQ8016
 	  - MSM8916
 
+config MXC_GPIO
+	bool "Freescale/NXP MXC UART driver"
+	help
+	  Support GPIO controllers on various i.MX platforms
+
 config OMAP_GPIO
 	bool "TI OMAP GPIO driver"
 	depends on ARCH_OMAP2PLUS