board/zynqmp/post-image.sh: Don't insist on a xilinx/ prefix for .dts files.

If using BR2_LINUX_KERNEL_CUSTOM_DTS_PATH to copy .dts files from
buildroot into the linux tree, these .dts files are copied to
arch/arm64/boot. Unfortunately, the post-image.sh script expects to find
them in arch/arm64/boot/xilinx.

This patch does not require the xilinx/ prefix to be present when
symlinking the device-tree to system.dtb where u-boot expects to find
it.

Signed-off-by: Graeme Smecher <gsmecher@threespeedlogic.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Neal Frager <neal.frager@amd.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>