gpio-rcar: Add pinctrl support

Register the GPIO pin range, and request and free GPIO pins using the
pinctrl API.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>