gpio-rcar: Make the platform data gpio_base field signed

The gpio_base field is used to specify the desired GPIO base for the
GPIO controller. The GPIO core can automatically allocate a GPIO number
range when the base is set to -1. To make this possible, make the field
signed.

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