dt-bindings: usb: fix reg-property port-number range

The USB hub port-number range for USB 2.0 is 1-255 and not 1-31 which
reflects an arbitrary limit set by the current Linux implementation.

Note that for USB 3.1 hubs the valid range is 1-15.

Increase the documented valid range in the binding to 255, which is the
maximum allowed by the specifications.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>