clocksource/drivers/pistachio: Convert to SPDX identifier

The licensing text references explicitely the COPYING file in the kernel
base directory, which is clearly GPL version 2 only.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20220510171254.843410802@linutronix.deSigned-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/timer-pistachio.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Pistachio clocksource based on general-purpose timers
  *
  * Copyright (C) 2015 Imagination Technologies
- *
- * This file is subject to the terms and conditions of the GNU General Public
- * License. See the file "COPYING" in the main directory of this archive
- * for more details.
  */
 
 #define pr_fmt(fmt) "%s: " fmt, __func__