arm: dts: kirkwood*.dts: use SPDX-License-Identifier for boards 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>
diff --git a/arch/arm/boot/dts/kirkwood-rs411.dts b/arch/arm/boot/dts/kirkwood-rs411.dts
index 02852b0..41fa63c 100644
--- a/arch/arm/boot/dts/kirkwood-rs411.dts
+++ b/arch/arm/boot/dts/kirkwood-rs411.dts
@@ -1,10 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * Andrew Lunn <andrew@lunn.ch>
  * Ben Peddell <klightspeed@killerwolves.net>
  *
- * 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.
  */
 
 /dts-v1/;