ARM: multi_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>