SPM: Add backend operation 'replying'

SFN and IPC have different message replying mechanisms:

- SFN replies to the client by SPM after messaging is performed;
  while IPC partition calls 'psa_reply' initiatively to reply to
  the client.

Abstract 'replying' to decouple the backend-specific operations
out of common FFM logic.

Change-Id: Ie28a0eaa1b37dfe458328dfb6ba298a0bf979c26
Signed-off-by: Ken Liu <Ken.Liu@arm.com>
diff --git a/secure_fw/spm/ffm/psa_api.c b/secure_fw/spm/ffm/psa_api.c
index 16e0d04..0190c02 100644
--- a/secure_fw/spm/ffm/psa_api.c
+++ b/secure_fw/spm/ffm/psa_api.c
@@ -763,11 +763,7 @@
         conn_handle->status = TFM_HANDLE_STATUS_IDLE;
     }
 
-    if (is_tfm_rpc_msg(msg)) {
-        tfm_rpc_client_call_reply(msg, ret);
-    } else {
-        thrd_wake_up(&msg->ack_evnt, ret);
-    }
+    backend_instance.replying(msg, ret);
 }
 
 void tfm_spm_partition_psa_notify(int32_t partition_id)