usb: misc: usb3503: Describe better how to bind clock to the hub

- refclk: Clock used for driving REFCLK signal (optional, if not provided

	the driver assumes that clock signal is always available, its

	rate is specified by REF_SEL pins and a value from the primary

	reference clock frequencies table is used)

	reference clock frequencies table is used). Use clocks and

	clock-names in order to assign it

- refclk-frequency: Frequency of the REFCLK signal as defined by REF_SEL

	pins (optional, if not provided, driver will not set rate of the

	REFCLK signal and assume that a value from the primary reference

		intn-gpios = <&gpx3 4 1>;

		reset-gpios = <&gpx3 5 1>;

		initial-mode = <1>;

		clocks = <&clks 80>;

		clock-names = "refclk";

	};