arm: dts: kirkwood*.dts: use SPDX-License-Identifier for board using GPL-2.0+

Follow the recent trend for the license description

Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>

arch/arm/boot/dts/kirkwood-ts419-6282.dts

@@ -1,12 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0+
 /*
  * Device Tree file for QNAP TS41X with 6282 SoC
  *
  * Copyright (C) 2013, Andrew Lunn <andrew@lunn.ch>
- *
- * 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.
  */
 
 /dts-v1/;