doc: add bindings document for altera fpga manager

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>;

	};