Silicon/Socionext/SynQuacer: use Generic Event Device for power button

Drop the description of the interrupt handling part of the GPIO
controller, and use a ACPI0013 Generic Event device instead to
handle the power button event. This way, an OS can handle these
events without the need for a SoC specific driver.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>