libmbedtls: refine mbedtls license header

New header format according to this:
https://optee.readthedocs.io/general/license_headers.html

Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Edison Ai <edison.ai@arm.com>
[jw: removed the now redundant SPDX-License-Identifier]
[jw: updated the link to readthredocs]
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/lib/libmbedtls/mbedtls/library/oid.c b/lib/libmbedtls/mbedtls/library/oid.c
index edea950..8719c5a 100644
--- a/lib/libmbedtls/mbedtls/library/oid.c
+++ b/lib/libmbedtls/mbedtls/library/oid.c
@@ -1,10 +1,10 @@
+// SPDX-License-Identifier: Apache-2.0
 /**
  * \file oid.c
  *
  * \brief Object Identifier (OID) database
  *
  *  Copyright (C) 2006-2015, ARM Limited, All Rights Reserved
- *  SPDX-License-Identifier: Apache-2.0
  *
  *  Licensed under the Apache License, Version 2.0 (the "License"); you may
  *  not use this file except in compliance with the License.