gpio: Add GPIO driver for Spreadtrum SC9860 platform

The Spreadtrum SC9860 platform GPIO controller contains 16 groups and
each group contains 16 GPIOs. Each GPIO can set input/output and has
the interrupt capability.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>