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

A V4L2 driver for Renesas R-Car MIPI CSI-2 receiver. The driver
supports the R-Car Gen3 SoCs where separate CSI-2 hardware blocks are
connected between the video sources and the video grabbers (VIN).

Driver is based on a prototype by Koji Matsuoka in the Renesas BSP.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>