feat(smccc): add vendor-specific el3 service

Add support for vendor-specific el3 service. SMCCC 1.5 introduces
support for vendor-specific EL3 monitor calls.

SMCCC Documentation reference:
https://developer.arm.com/docs/den0028/latest

Change-Id: Id8bc43842eecdb7a8a2ec7f31a631e88fe4fe0b4
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
diff --git a/docs/getting_started/rt-svc-writers-guide.rst b/docs/getting_started/rt-svc-writers-guide.rst
index 2383cee..4d4ec22 100644
--- a/docs/getting_started/rt-svc-writers-guide.rst
+++ b/docs/getting_started/rt-svc-writers-guide.rst
@@ -315,9 +315,17 @@
 the BL31 support for these services. Or a reference to the document that will
 provide this information....
 
+Additional References:
+----------------------
+
+#. :ref:`ARM SiP Services <arm sip services>`
+#. :ref:`Vendor Specific EL3 Monitor Service Calls`
+
 --------------
 
 *Copyright (c) 2014-2024, Arm Limited and Contributors. All rights reserved.*
 
 .. _SMCCC: https://developer.arm.com/docs/den0028/latest
 .. _PSCI: https://developer.arm.com/documentation/den0022/latest/
+.. _ARM SiP Services: arm-sip-service.rst
+.. _Vendor Specific EL3 Monitor Service Calls: ven-el3-service.rst