dt-bindings: display: renesas,rzg2l-du: Document RZ/G2UL DU bindings

Document DU found in RZ/G2UL SoC. The DU block is identical to RZ/G2L
SoC, but has only DPI interface.

While at it, add missing required property port@1 for RZ/G2L and RZ/V2L
SoCs. Currently there is no user for the DPI interface and hence there
won't be any ABI breakage for adding port@1 as required property for
RZ/G2L and RZ/V2L SoCs.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240822162320.5084-2-biju.das.jz@bp.renesas.com