drm: rcar-du: Add R8A7745 support

Add support for the R8A7745 DU (which is very similar to the R8A7794 DU);
it has 2 RGB outputs.

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