media: rcar-isp: Add family compatible for R-Car Gen4 family

Add the Gen4 family compatible. This will be used instead of a SoC
specific compatible for the new Gen4 SoC V4M. Two Gen4 boards (V3U and
V4H) have already been added prior and their bindings need to be kept
for backward compatibility.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>