ARM: imx_v6_v7_defconfig: Select CONFIG_MUX_MMIO

In order to get ADV7180 analog video decoder to probe on a Gateworks
GW51xx we need select CONFIG_MUX_MMIO, otherwise /dev/media node does
not get created.

Select CONFIG_MUX_MMIO, so that the ADV7180 driver can be correctly
probed.

Reported-by: Tim Harvey <tharvey@gateworks.com>
Suggested-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>