ARM: dts: imx7s: Move csi-mux to below root

ARM: dts: imx7s: Move csi-mux to below root

fsl,imx-iomuxc-gpr.yaml only contains the mux-controller but the actual
video-mux is not part of it. So move it below root node.
Fixes the dtbs_check warning:
arch/arm/boot/dts/nxp/imx/imx7s-mba7.dtb: iomuxc-gpr@30340000: 'csi-mux' does not match any of the regexes: 'pinctrl-[0-9]+'
  from schema $id: http://devicetree.org/schemas/soc/imx/fsl,imx-iomuxc-gpr.yaml#

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>