dt-bindings: of: irq: document properties for wakeup interrupt parent

Some interrupt controllers in a SoC, are always powered on and have a
select interrupts routed to them, so that they can wakeup the SoC from
suspend. Add wakeup-parent DT property to refer to these interrupt
controllers.

Change-Id: I3601e3a037250a77253dce281c5f503400a6dc26