gpio-rcar: document R8A77980 bindings

Renesas R-Car V3H (R8A77980) SoC also has the R-Car gen3 compatible GPIO
controllers, so document the SoC specific bindings.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>