staging: xillybus: Added documentation on device tree bindings

Signed-off-by: NEli Billauer <eli.billauer@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/staging/xillybus.txt

+* 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>;
+	} ;