GIC: Add API to get running priority

Document the API in separate platform interrupt controller API document.

Change-Id: If18f208e10a8a243f5c59d226fcf48e985941949
Co-authored-by: Yousuf A <yousuf.sait@arm.com>
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>
diff --git a/docs/porting-guide.rst b/docs/porting-guide.rst
index 6c07b2e..c0b173c 100644
--- a/docs/porting-guide.rst
+++ b/docs/porting-guide.rst
@@ -2289,6 +2289,10 @@
 GICv3 depending on the build flag ``FVP_USE_GIC_DRIVER`` (See FVP platform
 specific build options in `User Guide`_ for more details).
 
+See also: `Interrupt Controller Abstraction APIs`__.
+
+.. __: platform-interrupt-controller-API.rst
+
 Function : plat\_interrupt\_type\_to\_line() [mandatory]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -2636,7 +2640,7 @@
 
 --------------
 
-*Copyright (c) 2013-2016, ARM Limited and Contributors. All rights reserved.*
+*Copyright (c) 2013-2017, ARM Limited and Contributors. All rights reserved.*
 
 .. _Migration Guide: platform-migration-guide.rst
 .. _include/plat/common/platform.h: ../include/plat/common/platform.h