media: rcar-vin: remove generic gen3 compatible string

The compatible string "renesas,rcar-gen3-vin" was added before the
Gen3 driver code was added but it's not possible to use. Each SoC in the
Gen3 series require SoC specific knowledge in the driver to function.
Remove it before it is added to any device tree descriptions.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>