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>