dt-bindings: xilinx: Add description for ZynqMP

Record xlnx,zynqmp compatible string.
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/arm/xilinx.txt

@@ -5,3 +5,13 @@ shall have the following properties.
 
 Required root node properties:
     - compatible = "xlnx,zynq-7000";
+
+---------------------------------------------------------------
+
+Xilinx Zynq UltraScale+ MPSoC Platforms Device Tree Bindings
+
+Boards with ZynqMP SOC based on an ARM Cortex A53 processor
+shall have the following properties.
+
+Required root node properties:
+    - compatible = "xlnx,zynqmp";