irqchip/stm32: Fix copyright

Uniformize STMicroelectronics copyrights header and add SPDX identifier

CC: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>
Acked-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: jason@lakedaemon.net
Cc: marc.zyngier@arm.com
Cc: linux-arm-kernel@lists.infradead.org
Link: https://lkml.kernel.org/r/20171130084500.23439-1-benjamin.gaignard@st.com

drivers/irqchip/irq-stm32-exti.c

+// 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/bitops.h>