ARM: pxa: convert incorrect IRQ_TO_IRQ() to irq_to_gpio()

This fixes the failure to register the IRQ_RTCAlrm alarm as a wakeup
event.  It is misinterpreted as a gpio irq not a PWER bitmask. Fixed
this by converting the incorrect IRQ_TO_IRQ() to a correct version of
irq_to_gpio().

Reported-by: Nick Bane <nickbane1@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>