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 <b2441318>)
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm64/boot/dts/xilinx/Makefile

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

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

+// SPDX-License-Identifier: GPL-2.0+
 /*
  * clock specification for Xilinx ZynqMP ep108 development board
  *

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

+// SPDX-License-Identifier: GPL-2.0+
 /*
  * dts file for Xilinx ZynqMP ep108 development board
  *

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

+// SPDX-License-Identifier: GPL-2.0+
 /*
  * dts file for Xilinx ZynqMP
  *