drm: rcar-du: Add R8A7743 support

Add support for the R8A7743 DU (which is very similar to the R8A7791 DU);
it has 1 DPAD (RGB) output and 1 LVDS output.

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>