ARM: stm32: Fix copyright

Uniformize STMicroelectronics copyrights header
Add SPDX identifier

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
CC: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Acked-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/mach-stm32/board-dt.c

@@ -1,7 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * Copyright (C) Maxime Coquelin 2015
+ * Copyright (C) STMicroelectronics 2017
  * Author:  Maxime Coquelin <mcoquelin.stm32@gmail.com>
- * License terms:  GNU General Public License (GPL), version 2
  */
 
 #include <linux/kernel.h>