gpio: pmic_eic: Add edge trigger emulation for PMIC EIC

This patch will toggle the EIC level to emulate the edge trigger to
support PMIC EIC egdge trigger function, which is required by gpio-keys
driver.

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