Add SPDX lines to each imported TinyCrypt file

Out of the 6 tinycrypt files included in Mbed TLS, this commit adds SPDX lines
to each for the BSD 3 Clause licence.
diff --git a/include/tinycrypt/ecc.h b/include/tinycrypt/ecc.h
index 37a7ef1..55f1ea8 100644
--- a/include/tinycrypt/ecc.h
+++ b/include/tinycrypt/ecc.h
@@ -3,6 +3,8 @@
 /* Copyright (c) 2014, Kenneth MacKay
  * All rights reserved.
  *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
  *