arm64: zynqmp: Fix serial compatible string

Based on commit 65a2c14d

 ("dt-bindings: serial: convert Cadence UART
bindings to YAML") compatible string should look like differently that's
why fix it to be aligned with dt binding.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/89b36e0a6187cc6b05b27a035efdf79173bd4486.1628240307.git.michal.simek@xilinx.com