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>

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

@@ -0,0 +1,13 @@
+FPGA Bridge Device Tree Binding
+
+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:
+	fpga_bridge3: fpga-bridge@ffc25080 {
+		compatible = "altr,socfpga-fpga2sdram-bridge";
+		reg = <0xffc25080 0x4>;
+		bridge-enable = <0>;
+	};