gpio: Add Spreadtrum EIC driver support

The Spreadtrum digital-chip EIC controller has 4 sub-modules: debounce EIC,
latch EIC, async EIC and sync EIC, and each sub-module can has multiple
banks and each bank contains 8 EICs.

Each EIC can only be used as input mode, and has 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>