drm: rcar-du: Add support for R8A774E1 SoC

Hookup RZ/G2H (R8A774E1) to DU driver. R8A774E1 has one RGB output,
one LVDS output and one HDMI output.

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