dt-bindings: pinctrl: Add DT bindings for Renesas RZ/V2M pinctrl

Add device tree binding documentation and header file for Renesas
RZ/V2M pinctrl.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220624084833.22605-2-phil.edworthy@renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>