ITS: Implement ITS Services with static handle

Change Internal Trusted Storage services to one stateless service.

Change-Id: I92cad72e85255cf4970f314681e4c871f171b72f
Signed-off-by: Shawn Shan <Shawn.Shan@arm.com>
diff --git a/interface/include/tfm_its_defs.h b/interface/include/tfm_its_defs.h
index 852f8f4..a6591b1 100644
--- a/interface/include/tfm_its_defs.h
+++ b/interface/include/tfm_its_defs.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019, Arm Limited. All rights reserved.
+ * Copyright (c) 2019-2021, Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  *
@@ -15,6 +15,12 @@
 /* Invalid UID */
 #define TFM_ITS_INVALID_UID 0
 
+/* ITS message types that distinguish ITS services. */
+#define TFM_ITS_SET                1001
+#define TFM_ITS_GET                1002
+#define TFM_ITS_GET_INFO           1003
+#define TFM_ITS_REMOVE             1004
+
 #ifdef __cplusplus
 }
 #endif