drm: rcar-du: Add R8A7792 support

Add support for the R8A7792 DU; it has 2 DPAD (RGB) outputs.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>