gpio: eic: Add edge trigger emulation for EIC

The Spreadtrum debounce EIC and latch EIC can not support edge trigger,
but most GPIO users (like gpio-key driver) only use the edge trigger,
thus the EIC driver need add some support to emulate the edge trigger
to satisfy this requirement.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>