dt-bindings: fpga: Consolidate bridge properties

Consolidate bridge properties in a single file, instead of duplicating
the same optional property over and over again.

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