watchdog: stmp3xxx: Switch to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management.
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

drivers/watchdog/stmp3xxx_rtc_wdt.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Watchdog driver for the RTC based watchdog in STMP3xxx and i.MX23/28
  *
  * Author: Wolfram Sang <kernel@pengutronix.de>
  *
  * Copyright (C) 2011-12 Wolfram Sang, Pengutronix
- *
- * This program is free software; you can redistribute it and/or modify it
- * under the terms of the GNU General Public License version 2 as published by
- * the Free Software Foundation.
  */
 #include <linux/kernel.h>
 #include <linux/module.h>