drm: rcar-du: Add r8a77470 support

Add RZ/G1C (a.k.a. r8a77470) support to the R-Car DU driver.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>