dts: stmmac: Document the clocks property in the stmmac base document

	reset phandle is given

- max-frame-size:	Maximum Transfer Unit (IEEE defined MTU), rather

			than the maximum frame size.

- clocks: If present, the first clock should be the GMAC main clock,

  further clocks may be specified in derived bindings.

- clocks-names: One name for each entry in the clocks property, the

  first one should be "stmmaceth".

Examples:

		mac-address = [000000000000]; /* Filled in by U-Boot */

		max-frame-size = <3800>;

		phy-mode = "gmii";

		clocks = <&clock>;

		clock-names = "stmmaceth">;

	};