media: rcar-csi2: add Renesas R-Car MIPI CSI-2 receiver documentation

Documentation for Renesas R-Car MIPI CSI-2 receiver. The CSI-2 receivers
are located between the video sources (CSI-2 transmitters) and the video
grabbers (VIN) on Gen3 of Renesas R-Car SoC.

Each CSI-2 device is connected to more than one VIN device which
simultaneously can receive video from the same CSI-2 device. Each VIN
device can also be connected to more than one CSI-2 device. The routing
of which links are used is controlled by the VIN devices. There are only
a few possible routes which are set by hardware limitations, which are
different for each SoC in the Gen3 family.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>