devicetree: Add compatible string for Zynq Ultrascale+ MPSoC

Add "cdns,zynqmp-gem" to be used for Zynq Ultrascale+ MPSoC.

Signed-off-by: Harini Katakam <harinik@xilinx.com>
Reviewed-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>