dt-bindings: fpga: fix freeze controller compatible in region doc

Documentation/devicetree/bindings/fpga/fpga-region.txt has an error
regarding the freeze controller bridge, secifically:

 compatible = "altr,freeze-bridge";

The compatibility string should be "altr,freeze-bridge-controller"
as set forth in altera-freeze-bridge.txt.

Signed-off-by: Alan Tull <atull@kernel.org>
Reported-by: Dalon Westergreen <dalon.westergreen@intel.com>
Acked-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>