Add ability to handle ACPI events signalled by GPIO interrupts.

ACPI5 platforms can use GPIO signaled ACPI events. These GPIO interrupts are
handled by ACPI event methods which need to be called from the GPIO
controller's interrupt handler. acpi_gpio_request_interrupt() finds out which
gpio pins have acpi event methods and assigns interrupt handlers that calls
the acpi event methods for those pins.

Partially based on work by Rui Zhang <rui.zhang@intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>