drm: rcar-du: Add R8A774B1 support

Add support for the R8A774B1 DU (which is very similar to the R8A77965 DU
except that it lacks TCON and CMM); it has one RGB output, one LVDS output
and one HDMI output.

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>