gpio: Add Spreadtrum PMIC EIC driver support

The Spreadtrum PMIC EIC controller contains only one bank of debounce EIC,
and this bank contains 16 EICs. Each EIC can only be used as input mode,
as well as supporting the debounce and the capability to trigger interrupts
when detecting input signals.

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>