rtc: max6900: convert to SPDX identifier

Use SPDX-License-Identifier instead of a verbose license text.
Signed-off-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210707075804.337458-4-nobuhiro1.iwamatsu@toshiba.co.jp

drivers/rtc/rtc-max6900.c

+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * rtc class driver for the Maxim MAX6900 chip
  *
+ * Copyright (c) 2007 MontaVista, Software, Inc.
+ *
  * Author: Dale Farnsworth <dale@farnsworth.org>
  *
  * based on previously existing rtc class drivers
- *
- * 2007 (c) MontaVista, Software, Inc.  This file is licensed under
- * the terms of the GNU General Public License version 2.  This program
- * is licensed "as is" without any warranty of any kind, whether express
- * or implied.
  */
 
 #include <linux/module.h>