rtc: ds2404: 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-ds2404.c

-/*
- * Copyright (C) 2012 Sven Schnelle <svens@stackframe.org>
- *
- * 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.
- *
- */
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (C) 2012 Sven Schnelle <svens@stackframe.org>
 
 #include <linux/platform_device.h>
 #include <linux/module.h>