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>
diff --git a/include/plat/common/platform.h b/include/plat/common/platform.h
index 068d7aa..086e5e6 100644
--- a/include/plat/common/platform.h
+++ b/include/plat/common/platform.h
@@ -90,6 +90,7 @@
 void plat_ic_set_interrupt_pending(unsigned int id);
 void plat_ic_clear_interrupt_pending(unsigned int id);
 unsigned int plat_ic_set_priority_mask(unsigned int mask);
+unsigned int plat_ic_get_interrupt_id(unsigned int raw);
 
 /*******************************************************************************
  * Optional common functions (may be overridden)