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/include/lib/aarch64/arch.h b/include/lib/aarch64/arch.h
index f85e789..3fb6846 100644
--- a/include/lib/aarch64/arch.h
+++ b/include/lib/aarch64/arch.h
@@ -68,6 +68,7 @@
 #define ICC_CTLR_EL1    S3_0_C12_C12_4
 #define ICC_CTLR_EL3    S3_6_C12_C12_4
 #define ICC_PMR_EL1     S3_0_C4_C6_0
+#define ICC_RPR_EL1     S3_0_C12_C11_3
 #define ICC_IGRPEN1_EL3 S3_6_c12_c12_7
 #define ICC_IGRPEN0_EL1 S3_0_c12_c12_6
 #define ICC_HPPIR0_EL1  S3_0_c12_c8_2