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/flea3_defconfig b/configs/flea3_defconfig
index 9f9d8ee..069f442 100644
--- a/configs/flea3_defconfig
+++ b/configs/flea3_defconfig
@@ -19,6 +19,7 @@
 CONFIG_MTDIDS_DEFAULT="nand0=mxc_nand,nor0=physmap-flash.0"
 CONFIG_MTDPARTS_DEFAULT="mtdparts=mxc_nand:50m(root1),32m(rootfb),64m(pcache),64m(app1),10m(app2),-(spool);physmap-flash.0:512k(u-boot),64k(env1),64k(env2),3776k(kernel1),3776k(kernel2)"
 CONFIG_ENV_IS_IN_FLASH=y
+CONFIG_MXC_GPIO=y
 # CONFIG_MMC is not set
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_NAND=y