ARM: socfpga: add bindings document for fpga bridge drivers

Add bindings documentation for Altera SOCFPGA bridges:
 * fpga2sdram
 * fpga2hps
 * hps2fpga
 * lwhps2fpga
Signed-off-by: Alan Tull <atull@opensource.altera.com>
Signed-off-by: Matthew Gerlach <mgerlach@altera.com>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/fpga/altera-fpga2sdram-bridge.txt

+Altera FPGA To SDRAM Bridge Driver
+
+Required properties:
+- compatible		: Should contain "altr,socfpga-fpga2sdram-bridge"
+
+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>;
+	};

Documentation/devicetree/bindings/fpga/altera-hps2fpga-bridge.txt

+Altera FPGA/HPS Bridge Driver
+
+Required properties:
+- regs		: base address and size for AXI bridge module
+- compatible	: Should contain one of:
+		  "altr,socfpga-lwhps2fpga-bridge",
+		  "altr,socfpga-hps2fpga-bridge", or
+		  "altr,socfpga-fpga2hps-bridge"
+- resets	: Phandle and reset specifier for this bridge's reset
+- clocks	: Clocks used by this 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 its current state.
+
+Example:
+	fpga_bridge0: fpga-bridge@ff400000 {
+		compatible = "altr,socfpga-lwhps2fpga-bridge";
+		reg = <0xff400000 0x100000>;
+		resets = <&rst LWHPS2FPGA_RESET>;
+		clocks = <&l4_main_clk>;
+		bridge-enable = <0>;
+	};
+
+	fpga_bridge1: fpga-bridge@ff500000 {
+		compatible = "altr,socfpga-hps2fpga-bridge";
+		reg = <0xff500000 0x10000>;
+		resets = <&rst HPS2FPGA_RESET>;
+		clocks = <&l4_main_clk>;
+		bridge-enable = <1>;
+	};
+
+	fpga_bridge2: fpga-bridge@ff600000 {
+		compatible = "altr,socfpga-fpga2hps-bridge";
+		reg = <0xff600000 0x100000>;
+		resets = <&rst FPGA2HPS_RESET>;
+		clocks = <&l4_main_clk>;
+	};