dt-bindings: gpio: renesas, rcar-gpio: Add r8a774e1 support

Document Renesas RZ/G2H (R8A774E1) GPIO blocks compatibility within the
relevant dt-bindings. R8A774E1 GPIO module is identical to R-Car Gen3
family.

No driver change is needed due to the fallback compatible value
"renesas,rcar-gen3-gpio".

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@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>