staging: xillybus: Added documentation on device tree bindings

* Xillybus driver for generic FPGA interface

Required properties:

- compatible: Should be "xillybus,xillybus-1.00.a"

- reg: Address and length of the register set for the device

- interrupts: Contains one interrupt node, typically consisting of three cells.

- interrupt-parent: the phandle for the interrupt controller that

                    services interrupts for this device.

Optional properties:

- dma-coherent: Present if DMA operations are coherent

Example:

	xillybus@ff200400 {

		compatible = "xillybus,xillybus-1.00.a";

		reg = < 0xff200400 0x00000080 >;

		interrupts = < 0 40 1 >;

		interrupt-parent = <&intc>;

	} ;