drm: rcar-du: lvds: Add r8a774b1 support

The LVDS encoders on RZ/G2N SoC is similar to R-Car M3-N. Add support for
RZ/G2N (R8A774B1) SoC to the LVDS encoder driver.

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