ARM: dts: zynq: 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,zynq-gem compatible string.

Signed-off-by: Harini Katakam <harini.katakam@amd.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/7768d5d68fe38dd8e9300e9c6e09c228e79b2862.1672909533.git.michal.simek@amd.com