feat(drtm): update return code if secondary PE is not off

DRTM 1.0 specifies that if any secondary PEs are not off
during a dynamic launch the return code must be
SECONDARY_PE_NOT_OFF.

Signed-off-by: Stuart Yoder <stuart.yoder@arm.com>
Change-Id: Idcb1f3c60daa63a5bc994bdeacca8aab7066f628
diff --git a/services/std_svc/drtm/drtm_main.c b/services/std_svc/drtm/drtm_main.c
index 3acf683..6b99721 100644
--- a/services/std_svc/drtm/drtm_main.c
+++ b/services/std_svc/drtm/drtm_main.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022 Arm Limited. All rights reserved.
+ * Copyright (c) 2022-2024 Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier:    BSD-3-Clause
  *
@@ -211,7 +211,7 @@
 	running_on_single_core = psci_is_last_on_cpu_safe();
 	if (!running_on_single_core) {
 		ERROR("DRTM: invalid launch due to non-boot PE not being turned off\n");
-		return DENIED;
+		return SECONDARY_PE_NOT_OFF;
 	}
 
 	return SUCCESS;