usb: dwc3: Support option to disable USB2 LPM

Support the option to disable USB2 LPM. Set xhci "usb2-lpm-disable"
property via "snps,usb2-lpm-disable" property.

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