arm64: 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/arm64/configs/defconfig

@@ -500,6 +500,7 @@ CONFIG_GPIO_ALTERA=m
 CONFIG_GPIO_DWAPB=y
 CONFIG_GPIO_MB86S7X=y
 CONFIG_GPIO_MPC8XXX=y
+CONFIG_GPIO_MXC=y
 CONFIG_GPIO_PL061=y
 CONFIG_GPIO_RCAR=y
 CONFIG_GPIO_UNIPHIER=y