dt-bindings: gpio: rcar: Add DT binding for r8a774b1

Document Renesas' RZ/G2N (R8A774B1) GPIO blocks compatibility within the
relevant dt-bindings.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>