drm: rcar-du: lvds: Add r8a774a1 support

The LVDS encoders on RZ/G2M SoC is similar to R-Car M3-W. Add support for
RZ/G2M (R8A774A1) SoC to the LVDS encoder driver.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>