usb: dwc3: Add a property to disable USB2 LPM

Add an option to disable USB2 LPM from host. There maybe cases where the
user does not want to enable USB2 LPM (e.g. USB2 LPM is broken).

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>