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/drivers/arm/gicv3.h b/include/drivers/arm/gicv3.h
index b2e4d4c..f2a5371 100644
--- a/include/drivers/arm/gicv3.h
+++ b/include/drivers/arm/gicv3.h
@@ -245,6 +245,9 @@
 #define GICR_NUM_REGS(reg_name)	\
 	DIV_ROUND_UP_2EVAL(TOTAL_PCPU_INTR_NUM, (1 << reg_name ## _SHIFT))
 
+/* Interrupt ID mask for HPPIR, AHPPIR, IAR and AIAR CPU Interface registers */
+#define INT_ID_MASK	0xffffff
+
 /*******************************************************************************
  * This structure describes some of the implementation defined attributes of the
  * GICv3 IP. It is used by the platform port to specify these attributes in order