rtc: tx4939: convert to SPDX identifier

Use SPDX-License-Identifier instead of a verbose license text.

The original text refers to the COPYING file in the main directory which is
GPL v2 only so also update MODULE_LICENSE() to "GPL v2" instead of "GPL".
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-tx4939.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * TX4939 internal RTC driver
  * Based on RBTX49xx patch from CELF patch archive.
  *
- * This file is subject to the terms and conditions of the GNU General Public
- * License.  See the file "COPYING" in the main directory of this archive
- * for more details.
- *
  * (C) Copyright TOSHIBA CORPORATION 2005-2007
  */
 #include <linux/rtc.h>
@@ -310,5 +307,5 @@ module_platform_driver_probe(tx4939_rtc_driver, tx4939_rtc_probe);
 
 MODULE_AUTHOR("Atsushi Nemoto <anemo@mba.ocn.ne.jp>");
 MODULE_DESCRIPTION("TX4939 internal RTC driver");
-MODULE_LICENSE("GPL");
+MODULE_LICENSE("GPL v2");
 MODULE_ALIAS("platform:tx4939rtc");