arm64: zynqmp: Add SPDX license identifier

Add SPDX identifier as was done by for example by:
"License cleanup: add SPDX GPL-2.0 license identifier to files with no
license" (commit <b24413180f>)

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

arch/arm64/boot/dts/xilinx/Makefile

@@ -1 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0
 dtb-$(CONFIG_ARCH_ZYNQMP) += zynqmp-ep108.dtb

arch/arm64/boot/dts/xilinx/zynqmp-ep108-clk.dtsi

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0+
 /*
  * clock specification for Xilinx ZynqMP ep108 development board
  *

arch/arm64/boot/dts/xilinx/zynqmp-ep108.dts

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0+
 /*
  * dts file for Xilinx ZynqMP ep108 development board
  *

arch/arm64/boot/dts/xilinx/zynqmp.dtsi

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0+
 /*
  * dts file for Xilinx ZynqMP
  *