rtc: ds1672: 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-ds1672.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * An rtc/i2c driver for the Dallas DS1672
  * Copyright 2005-06 Tower Technologies
  *
  * Author: Alessandro Zummo <a.zummo@towertech.it>
- *
- * 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/i2c.h>