dt-bindings: gpio: rcar: Add r8a774a1 (RZ/G2M) support

Document Renesas' RZ/G2M (R8A774A1) GPIO blocks compatibility within the
relevant dt-bindings.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>