GIC: Introduce API to get interrupt ID

GIC: Introduce API to get interrupt ID

Acknowledging interrupt shall return a raw value from the interrupt
controller in which the actual interrupt ID may be encoded. Add a
platform API to extract the actual interrupt ID from the raw value
obtained from interrupt controller.

Document the new function. Also clarify the semantics of interrupt
acknowledge.

Change-Id: I818dad7be47661658b16f9807877d259eb127405
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>