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/mx35pdk_defconfig b/configs/mx35pdk_defconfig
index 61d2013..e550d40 100644
--- a/configs/mx35pdk_defconfig
+++ b/configs/mx35pdk_defconfig
@@ -23,6 +23,7 @@
 CONFIG_EFI_PARTITION=y
 # CONFIG_PARTITION_UUIDS is not set
 CONFIG_ENV_IS_IN_FLASH=y
+CONFIG_MXC_GPIO=y
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_NAND=y
 CONFIG_NAND_MXC=y