ARM: multi_v5_defconfig: Select CONFIG_GPIO_MXC

Since commit 12d16b39 ("gpio: mxc: Support module build") the
CONFIG_GPIO_MXC option needs to be explicitly selected.

Select it to avoid boot issues on imx25/imx27 due to the lack of the
GPIO driver.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>