dt-bindings: fpga: Add bindings for ZynqMP fpga driver

Add documentation to describe Xilinx ZynqMP fpga driver
bindings.

Signed-off-by: Nava kishore Manne <nava.manne@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Alan Tull <atull@kernel.org>
Acked-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>