GIC: Add API to set interrupt routing

SPIs can be routed to either a specific PE, or to any one of all
available PEs.

API documentation updated.

Change-Id: I28675f634568aaf4ea1aa8aa7ebf25b419a963ed
Co-authored-by: Yousuf A <yousuf.sait@arm.com>
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>
diff --git a/include/drivers/arm/gic_common.h b/include/drivers/arm/gic_common.h
index f4e2c52..14979cc 100644
--- a/include/drivers/arm/gic_common.h
+++ b/include/drivers/arm/gic_common.h
@@ -74,6 +74,7 @@
 #define ISACTIVER_SHIFT		5
 #define ICACTIVER_SHIFT		ISACTIVER_SHIFT
 #define IPRIORITYR_SHIFT	2
+#define ITARGETSR_SHIFT		2
 #define ICFGR_SHIFT		4
 #define NSACR_SHIFT		4