dt-bindings: gpio: Add Spreadtrum GPIO controller documentation

This patch adds the device tree bindings for the Spreadtrum
GPIO controller. The gpios will be supported by the GPIO
generic library.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>