Implement FFA_YIELD ABI

This change adds the missing FFA ABI and updates the documentation.

Change-Id: Ia302daf011e58984154ea33760a1912532d0753c
Signed-off-by: Gabor Toth <gabor.toth2@arm.com>
diff --git a/docs/uml/libsp_functions.puml b/docs/uml/libsp_functions.puml
index b46cc01..538d573 100644
--- a/docs/uml/libsp_functions.puml
+++ b/docs/uml/libsp_functions.puml
@@ -25,6 +25,7 @@
 
     .. CPU cycle management interfaces ..
     + ffa_msg_wait()
+    + ffa_yield()
 
     .. Messaging interfaces ..
     + ffa_msg_send_direct_req()