gpio: rcar: add R8A7792 support

Renesas  R8A7792 SoC is a member of the R-Car gen2 family, add support for
its GPIO controllers.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>