rtc: mc13xxx: convert to SPDX identifier

Use SPDX-License-Identifier instead of a verbose license text.
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-mc13xxx.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Real Time Clock driver for Freescale MC13XXX PMIC
  *
  * (C) 2009 Sascha Hauer, Pengutronix
  * (C) 2009 Uwe Kleine-Koenig, 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/mfd/mc13xxx.h>