ARM: imx_v6_v7_defconfig: Build in CONFIG_GPIO_MXC by default

i.MX SoC GPIO driver provides the basic functions of GPIO pin operations
and IRQ operations, it is now changed from "def_bool y" to "tristate", so
it should be explicitly enabled to make sure all consumers work normally.
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/configs/imx_v6_v7_defconfig

@@ -217,6 +217,7 @@ CONFIG_GPIO_PCA953X=y
 CONFIG_GPIO_PCF857X=y
 CONFIG_GPIO_STMPE=y
 CONFIG_GPIO_74X164=y
+CONFIG_GPIO_MXC=y
 CONFIG_POWER_RESET=y
 CONFIG_POWER_RESET_SYSCON=y
 CONFIG_POWER_RESET_SYSCON_POWEROFF=y