genirq/irq_sim: Use the SPDX license identifier in the header

Use C-style comment for the identifier as per
Documentation/process/license-rules.rst and remove the license boilerplate.
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20180426200747.8344-2-brgl@bgdev.pl

include/linux/irq_sim.h

-#ifndef _LINUX_IRQ_SIM_H
-#define _LINUX_IRQ_SIM_H
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
- * Copyright (C) 2017 Bartosz Golaszewski <brgl@bgdev.pl>
- *
- * This program is free software; you can redistribute  it and/or modify it
- * under  the terms of  the GNU General  Public License as published by the
- * Free Software Foundation;  either version 2 of the  License, or (at your
- * option) any later version.
+ * Copyright (C) 2017-2018 Bartosz Golaszewski <brgl@bgdev.pl>
  */
 
+#ifndef _LINUX_IRQ_SIM_H
+#define _LINUX_IRQ_SIM_H
+
 #include <linux/irq_work.h>
 #include <linux/device.h>