arm64: zynqmp: Use SPDX license with dc4

Just header change.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm/dts/zynqmp-zc1751-xm018-dc4.dts

@@ -5,10 +5,7 @@
  *
  * Michal Simek <michal.simek@xilinx.com>
  *
- * This program is free software; you can redistribute it and/or
+ * SPDX-License-Identifier:	GPL-2.0+
- * 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/;