ARM: dts: imx7: Group mipi_csi 'port' children in a 'ports' node

The nxp,imx7-mipi-csi2.yaml binding requires ports to be grouped in a
ports node, as multiple ports are present. Fix imx7s.dtsi and the only
board file that references the mipi_csi ports.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>