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/drivers/arm/gic/v2/gicv2_private.h b/drivers/arm/gic/v2/gicv2_private.h
index 91ab43a..7703cbe 100644
--- a/drivers/arm/gic/v2/gicv2_private.h
+++ b/drivers/arm/gic/v2/gicv2_private.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2015-2017, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -80,6 +80,11 @@
 	return mmio_read_32(base + GICC_IIDR);
 }
 
+static inline unsigned int gicc_read_rpr(uintptr_t base)
+{
+	return mmio_read_32(base + GICC_RPR);
+}
+
 /*******************************************************************************
  * GIC CPU interface accessors for writing entire registers
  ******************************************************************************/