drm: rcar-du: lvds: add R8A774C0 support

The LVDS implementation on the RZ/G2E (a.k.a. R8A774C0) is very similar
to the one found on R-Car E3 (a.k.a. R8A77990), therefore add RZ/G2E
LVDS support to the LVDS encoder driver in a similar fashion to what is
done for R-Car E3.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>