doc: add bindings document for altera fpga manager

New bindings document for Altera fpga manager.
Signed-off-by: Alan Tull <atull@opensource.altera.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/fpga/altera-socfpga-fpga-mgr.txt

+Altera SOCFPGA FPGA Manager
+
+Required properties:
+- compatible : should contain "altr,socfpga-fpga-mgr"
+- reg        : base address and size for memory mapped io.
+               - The first index is for FPGA manager register access.
+               - The second index is for writing FPGA configuration data.
+- interrupts : interrupt for the FPGA Manager device.
+
+Example:
+
+	hps_0_fpgamgr: fpgamgr@0xff706000 {
+		compatible = "altr,socfpga-fpga-mgr";
+		reg = <0xFF706000 0x1000
+		       0xFFB90000 0x1000>;
+		interrupts = <0 175 4>;
+	};