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/apf27_defconfig b/configs/apf27_defconfig
index 6949b71..c2faead 100644
--- a/configs/apf27_defconfig
+++ b/configs/apf27_defconfig
@@ -33,6 +33,7 @@
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_FPGA_XILINX=y
 CONFIG_MMC_MXC=y
+CONFIG_MXC_GPIO=y
 CONFIG_NAND=y
 CONFIG_NAND_MXC=y
 CONFIG_SYS_NAND_BUSWIDTH_16BIT=y