dt-bindings: display: bridge: Document RZ/G2L MIPI DSI TX bindings

The RZ/G2L MIPI DSI TX is embedded in the Renesas RZ/G2L family SoC's. It
can operate in DSI mode, with up to four data lanes.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>