kconfig: Convert CONFIG_MXS_GPIO to Kconfig

This converts the following to Kconfig:
   CONFIG_MXS_GPIO

Travis-CI: https://travis-ci.org/lmajewski/u-boot-dfu/builds/571260789

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index 7d9c97f..f2dabb5 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -139,6 +139,11 @@
 	help
 	  Support GPIO controllers on various i.MX platforms
 
+config MXS_GPIO
+	bool "Freescale/NXP MXS GPIO driver"
+	help
+	  Support GPIO controllers on i.MX23 and i.MX28 platforms
+
 config OMAP_GPIO
 	bool "TI OMAP GPIO driver"
 	depends on ARCH_OMAP2PLUS