gpio: add gpio offset in gpio range cells property

Add gpio offset into "gpio-range-cells" property. It's used to support
sparse pinctrl range in gpio chip.

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>