dt-bindings: gpio: Add Spreadtrum EIC controller documentation

This patch adds the device tree bindings for the Spreadtrum EIC
controller. The EIC can be seen as a special type of GPIO, which
can only be used as input mode.

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