Cactus: FFA Version Test

Added simple test of the FFA interface to cactus:
- FFA version ABI helper;
- New test file for test to FFA ABIs;
- Invoking "ffa_tests" from cactus main.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I8c8445ad6d9da79f1880d143836e7a6da68eaff7
diff --git a/spm/cactus/cactus_tests.h b/spm/cactus/cactus_tests.h
index f4bcb7e..23586f5 100644
--- a/spm/cactus/cactus_tests.h
+++ b/spm/cactus/cactus_tests.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017-2018, Arm Limited. All rights reserved.
+ * Copyright (c) 2017-2020, Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -12,6 +12,11 @@
  */
 
 /*
+ * Test to FFA interfaces.
+ */
+void ffa_tests(void);
+
+/*
  * Test other things like the version number returned by SPM.
  */
 void misc_tests(void);