arm64: zynqmp: Create fdtfile from compatible string

distro boot expects that fdtfile name is setup for alternative DTB.
Create this file based on the first platform compatible string.
This should ensure that one rootfs can store multiple DTBs for different
boards.
Reflect structure which is used in Linux kernel. It means dtbs are
strored in xilinx folder.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Alexander Graf <agraf@suse.de>