media: rcar-csi2: Add r8a779a0 support

Add support for the R-Car V3U (r8a779a0) to the driver. The V3U have the
CSI-2 modules connected to ISPs instead of directly to the R-Car VIN DMA
engines.

The ISP performs channel selection based on CSI-2 VC/DT pairs and routes
the video data. This requires the R-Car CSI-2 media entity to modeled
differently then on other SoCs as it on the V3U only have a single
source pad connected to the ISP.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>