net: ethoc: document OF bindings

* OpenCores MAC 10/100 Mbps

Required properties:

- compatible: Should be "opencores,ethoc".

- reg: two memory regions (address and length),

  first region is for the device registers and descriptor rings,

  second is for the device packet memory.

- interrupts: interrupt for the device.

Optional properties:

- clocks: phandle to refer to the clk used as per

  Documentation/devicetree/bindings/clock/clock-bindings.txt

Examples:

	enet0: ethoc@fd030000 {

		compatible = "opencores,ethoc";

		reg = <0xfd030000 0x4000 0xfd800000 0x4000>;

		interrupts = <1>;

		local-mac-address = [00 50 c2 13 6f 00];

		clocks = <&osc>;

        };