board/versal/post-image.sh: do not require 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.

It is effectively applying the below patch for zynqmp to versal as well:
https://patchwork.ozlabs.org/project/buildroot/patch/20230201195956.1758827-1-gsmecher@threespeedlogic.com/



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