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: NAlan Tull <atull@opensource.altera.com>
Signed-off-by: NMatthew Gerlach <mgerlach@opensource.altera.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/fpga/altera-freeze-bridge.txt

+Altera Freeze Bridge Controller Driver
+
+The Altera Freeze Bridge Controller manages one or more freeze bridges.
+The controller can freeze/disable the bridges which prevents signal
+changes from passing through the bridge.  The controller can also
+unfreeze/enable the bridges which allows traffic to pass through the
+bridge normally.
+
+Required properties:
+- compatible		: Should contain "altr,freeze-bridge-controller"
+- regs			: base address and size for freeze bridge module
+
+Optional properties:
+- bridge-enable		: 0 if driver should disable bridge at startup
+			  1 if driver should enable bridge at startup
+			  Default is to leave bridge in current state.
+
+Example:
+	freeze-controller@100000450 {
+		compatible = "altr,freeze-bridge-controller";
+		regs = <0x1000 0x10>;
+		bridge-enable = <0>;
+	};