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>
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v2:
 * Moved gpio-controller to required
 * Wrote a better description of what the sub-nodes are for
 * Added pinmux property description
 * Changed macro RZA2_PIN_ID to RZA2_PIN
2 files changed