pinctrl: st: Add software edge trigger interrupt support

ST pin controller does not have hardware support for detecting edge
triggered interrupts, It only has level triggering support.
This patch attempts to fake up edge triggers from hw level trigger
support in software. With this facility now the gpios can be easily used
for keypads, otherwise it would be difficult for drivers like keypads to
work with level trigger interrupts.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>