ARM: dts: imx7s: add multiplexer controls

The IOMUXC General Purpose Register has bitfield to control video bus
multiplexer to control the CSI input between the MIPI-CSI2 and parallel
interface. Add that register and mask.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>