drm: rcar-du: lvds: Add r8a7742 support

The LVDS encoders on RZ/G1H SoC is identical to the R-Car Gen2 family. Add
support for RZ/G1H (R8A7742) SoC to the LVDS encoder 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>