gpio: sch: Hook into ACPI GPE handler to catch GPIO edge events

Neither the ACPI description on Intel Minnowboard (v1) platform provides
the required information to establish a generic handling nor the hardware
capable of doing it. According to the data sheet the hardware can generate
SCI events. Therefore, we need to hook from the driver into GPE handler of
the ACPI subsystem in order to catch and report GPIO-related events.

Validated on the Inlel Minnowboard (v1) platform and Intel Galileo Gen 2.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>