media: dt-bindings: rcar-vin: Document compatibility string for R8A77470

When adding the compatibility string for R8A77470 (RZ/G1C) to the
rcar-vin driver the string was never documented in the bindings, add it
now.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>