Adding SPDX identifier

Author: Ashok Rao

targets/TARGET_STM/TARGET_STM32F4/TARGET_STM32F429xI/TARGET_MTB_STM_S2LP/system_clock.c

@@ -1,6 +1,7 @@
 /* mbed Microcontroller Library
 * Copyright (c) 2006-2017 ARM Limited
-*
+* SPDX-License-Identifier: BSD-3-Clause
+
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at