Core: Add PSA_DOORBELL signal to assigned_signals

As PSA_DOORBELL is also a valid secure partition signal, add it
to assigned_signals.

Change-Id: I21d3099556557ce49dcdca938250a2ac4ad98d0e
Signed-off-by: Shawn Shan <shawn.shan@arm.com>
diff --git a/secure_fw/spm/spm_api_ipc.c b/secure_fw/spm/spm_api_ipc.c
index e126b7e..b36887f 100644
--- a/secure_fw/spm/spm_api_ipc.c
+++ b/secure_fw/spm/spm_api_ipc.c
@@ -557,6 +557,9 @@
             continue;
         }
 
+        /* Add PSA_DOORBELL signal to assigned_signals */
+        partition->runtime_data.assigned_signals |= PSA_DOORBELL;
+
         /* TODO: This can be optimized by generating the assigned signal
          *       in code generation time.
          */