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/configs/mx7ulp_evk_plugin_defconfig b/configs/mx7ulp_evk_plugin_defconfig
index cb66bca..9d323cf 100644
--- a/configs/mx7ulp_evk_plugin_defconfig
+++ b/configs/mx7ulp_evk_plugin_defconfig
@@ -13,6 +13,7 @@
 CONFIG_DM=y
 CONFIG_DM_GPIO=y
 CONFIG_IMX_RGPIO2P=y
+# CONFIG_MXC_GPIO is not set
 CONFIG_DM_I2C=y
 CONFIG_DM_MMC=y
 CONFIG_PINCTRL=y