gpio-rcar: document R8A77970 bindings

Renesas R-Car V3M (R8A77970) 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>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>