serial: zynq: Add missing xlnx,zynqmp-uart compatible

Based on Linux kernel DT binding xlnx,zynqmp-uart is another compatible
string which can be used for this driver. That's why also list it here.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/959a4cbbdd06a7fca2d9251ff0f535863a562b96.1641905717.git.michal.simek@xilinx.com