SST: Add uniform secure functions support

This patch adds the uniform secure functions support in SST.

Change-Id: I52475b43435ed3b6d289651ea777de9483116b7e
Signed-off-by: Marc Moreno <marc.morenoberengue@arm.com>
diff --git a/interface/include/tfm_sst_defs.h b/interface/include/tfm_sst_defs.h
index a008c73..1d54043 100644
--- a/interface/include/tfm_sst_defs.h
+++ b/interface/include/tfm_sst_defs.h
@@ -55,6 +55,9 @@
     TFM_SST_ERR_INVALID                                                        \
 )
 
+/* Invalid UID */
+#define TFM_SST_INVALID_UID 0
+
 #ifdef __cplusplus
 }
 #endif