add bindings document for altera freeze bridge

Add bindings document for the Altera Freeze Bridge.  A Freeze
Bridge is used to gate traffic to/from a region of a FPGA
such that that region can be reprogrammed.  The Freeze Bridge
exist in FPGA fabric that is not currently being reconfigured.

Signed-off-by: Alan Tull <atull@opensource.altera.com>
Signed-off-by: Matthew Gerlach <mgerlach@opensource.altera.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>