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/components/index.rst b/docs/components/index.rst
index 30d80fc..36ab6fa 100644
--- a/docs/components/index.rst
+++ b/docs/components/index.rst
@@ -26,3 +26,4 @@
    realm-management-extension
    rmm-el3-comms-spec
    granule-protection-tables-design
+   ven-el3-service