refactor(ff-a): replace `plat_ffa_` prefix for public functions

Replace `plat_ffa_` prefix with `ffa_{file_name}_` prefix. Only do so
for public functions to avoid unnecessary churn (private functions are
renamed in the next commit).

Change-Id: I847c218f134a2519a45a3af1d6c4b953db9c1cbb
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/inc/hf/ffa/direct_messaging.h b/inc/hf/ffa/direct_messaging.h
index 968a2e9..b103112 100644
--- a/inc/hf/ffa/direct_messaging.h
+++ b/inc/hf/ffa/direct_messaging.h
@@ -36,7 +36,10 @@
 void ffa_direct_msg_unwind_call_chain_ffa_direct_resp(
 	struct vcpu_locked current_locked, struct vcpu_locked next_locked);
 
-bool plat_ffa_handle_framework_msg(struct ffa_value args,
-				   struct ffa_value *ret);
+bool ffa_direct_msg_handle_framework_msg(struct ffa_value args,
+					 struct ffa_value *ret);
 
-bool plat_ffa_is_spmd_lp_id(ffa_id_t vm_id);
+bool ffa_direct_msg_precedes_in_call_chain(struct vcpu_locked current_locked,
+					   struct vcpu_locked target_locked);
+
+bool ffa_direct_msg_is_spmd_lp_id(ffa_id_t vm_id);