rtc: dm355evm: 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-dm355evm.c

+// SPDX-License-Identifier: GPL-2.0+
 /*
  * rtc-dm355evm.c - access battery-backed counter in MSP430 firmware
  *
  * Copyright (c) 2008 by David Brownell
- *
- * This program is free software; you can redistribute it and/or
- * modify it under the terms of the GNU General Public License
- * as published by the Free Software Foundation; either version
- * 2 of the License, or (at your option) any later version.
  */
 #include <linux/kernel.h>
 #include <linux/init.h>