feat(sdei): add a function to return total number of events registered

This patch adds a public API to return the total number of registered
events. The purpose of this is primarily for DRTM to ensure that no
SDEI event can interfere with a dynamic launch.

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: I1d1cba2da7d5566cc340620ee1ce7d7844740b86
diff --git a/include/services/sdei.h b/include/services/sdei.h
index 063ed6f..c12a182 100644
--- a/include/services/sdei.h
+++ b/include/services/sdei.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017-2020, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2022, Arm Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -137,4 +137,7 @@
 /* Public API to dispatch an event to Normal world */
 int sdei_dispatch_event(int ev_num);
 
+/* Public API to check how many SDEI events are registered. */
+int sdei_get_registered_event_count(void);
+
 #endif /* SDEI_H */