dt-bindings: pinctrl: Add RZ/A2 pinctrl and GPIO

Add device tree binding documentation and header file for Renesas R7S9210
(RZ/A2) SoCs.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>