arm64: dts: zynqmp: Add xlnx prefix to GEM compatible string

cdns,zynq/zynqmp/versal-gem was recently deprecated in Linux in
favour of xlnx prefix. Add this new compatible string and remove
the existing cdns,zynqmp-gem compatible string.

Signed-off-by: Harini Katakam <harini.katakam@amd.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/578a4fcbb4143888af954996a45f5e1110e0ee50.1672909426.git.michal.simek@amd.com