ARM: dts: imx7s: Add video mux, csi and mipi_csi

Add device tree nodes for csi, video multiplexer and mipi-csi.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>