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

Signed-off-by: Michael Trimarchi <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/usb/usb3503.txt

···
 - 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";
 	};