arm64: xilinx: Use zynqmp prefix for SOM dt overlays

U-Boot is using zynqmp- prefix to compose DT name for board detection
that's why also generate DT in this format in the kernel.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/49c1b8c992929610ba17b9c6edf5d40d4b28d2ed.1683033163.git.michal.simek@amd.com