dt-bindings: display: rcar-du: Document R8A774[35] DU

Add device tree bindings for r8a7743 and r8a7745 DUs.
r8a7743 DU is similar to the one from r8a7791, r8a7745 DU is similar
to the one from r8a7794.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[Don't reference R8A779[0123456] and R8A774[35] explicitly]
[Number all DPAD, HDMI and LVDS ports]
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>