dt-bindings: display: renesas,du: Provide bindings for r8a779a0

Extend the Renesas DU display bindings to support the r8a779a0 V3U.

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>