Core: Rename and move __cmse_secure_gateway macro

-- The __cmse_secure_gateway macro is renamed to better
   reflect its scope to __tfm_secure_gateway_attributes__
-- The macro is moved to tfm_secure_api.h to allow it
   to be visible to all veneer definitions, and it's
   removed from the corresponding declarations.

Change-Id: I8e8151d4af6f4f888e353c69ed9a8264794a88c3
Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>
diff --git a/interface/include/tfm_sst_api.h b/interface/include/tfm_sst_api.h
index 3cdb64e..66865ac 100644
--- a/interface/include/tfm_sst_api.h
+++ b/interface/include/tfm_sst_api.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017, Arm Limited. All rights reserved.
+ * Copyright (c) 2017 - 2018, Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  *
@@ -12,6 +12,8 @@
 extern "C" {
 #endif
 
+#include "tfm_sst_defs.h"
+
 /**
  * \brief Gets handler for the given asset UUID. If an asset is deleted, the
  *        linked asset handle reference is no longer valid and will give