drm: rcar-du: Add r8a7742 support

Add direct support for the r8a7742 (RZ/G1H).

The RZ/G1H shares a common, compatible configuration with the r8a7790
(R-Car H2) so that device info structure is reused, the only difference
being TCON is unsupported on RZ/G1H (Currently unsupported by the driver).

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Marian-Cristian Rotariu <marian-cristian.rotariu.rb@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>