devicetree: bindings: uart: Add new compatible string for ZynqMP

This patch Adds the new compatible string for ZynqMP SoC.
Signed-off-by: Nava kishore Manne <navam@xilinx.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/cdns,uart.txt

 Binding for Cadence UART Controller
 
 Required properties:
-- compatible : should be "cdns,uart-r1p8", or "xlnx,xuartps"
+- compatible :
+  Use "xlnx,xuartps","cdns,uart-r1p8" for Zynq-7xxx SoC.
+  Use "xlnx,zynqmp-uart","cdns,uart-r1p12" for Zynq Ultrascale+ MPSoC.
 - reg: Should contain UART controller registers location and length.
 - interrupts: Should contain UART controller interrupts.
 - clocks: Must contain phandles to the UART clocks