ARM: dts: imx: Switch NXP boards to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6dl-sabresd.dts

@@ -1,10 +1,6 @@
-/*
- * Copyright (C) 2013 Freescale Semiconductor, Inc.
- *
- * 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) 2013 Freescale Semiconductor, Inc.
 
 /dts-v1/;