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/mx31pdk_defconfig b/configs/mx31pdk_defconfig
index ea6daf2..e4e955c 100644
--- a/configs/mx31pdk_defconfig
+++ b/configs/mx31pdk_defconfig
@@ -14,6 +14,7 @@
 CONFIG_CMD_PING=y
 CONFIG_CMD_DATE=y
 CONFIG_ENV_IS_IN_NAND=y
+CONFIG_MXC_GPIO=y
 # CONFIG_MMC is not set
 CONFIG_NAND=y
 CONFIG_NAND_MXC=y