gpio: rcar: Add optional functional clock to bindings

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: linux-gpio@vger.kernel.org
Cc: devicetree@vger.kernel.org
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>