usb: misc: usb3503: Don't specify all DT properties as required

The binding document says that all properties are required but in fact
almost all are optional (and should be) - update the document to reflect
this.

Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/usb3503.txt

@@ -3,6 +3,8 @@ SMSC USB3503 High-Speed Hub Controller
 Required properties:
 - compatible: Should be "smsc,usb3503" or "smsc,usb3503a".
 - reg: Specifies the i2c slave address, it should be 0x08.
+
+Optional properties:
 - connect-gpios: Should specify GPIO for connect.
 - disabled-ports: Should specify the ports unused.
 	'1' or '2' or '3' are availe for this property to describe the port