ARM: dts: imx6qdl: Add video multiplexers, mipi_csi, and their connections

This patch adds the device tree graph connecting the input multiplexers
to the IPU CSIs and the MIPI-CSI2 gasket on i.MX6. The MIPI_IPU
multiplexers are added as children of the iomuxc-gpr syscon device node.
On i.MX6Q/D two two-input multiplexers in front of IPU1 CSI0 and IPU2
CSI1 allow to select between CSI0/1 parallel input pads and the MIPI
CSI-2 virtual channels 0/3.
On i.MX6DL/S two five-input multiplexers in front of IPU1 CSI0 and IPU1
CSI1 allow to select between CSI0/1 parallel input pads and any of the
four MIPI CSI-2 virtual channels.

Changes from Steve Longerbeam:
- Removed some dangling/unused endpoints (ipu2_csi0_from_csi2ipu)
- Renamed the mipi virtual channel endpoint labels, from "mipi_csiX_..."
  to "mipi_vcX...".
- Added input endpoint anchors to the video muxes for the connections
  from parallel sensors.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>