usb: dwc3: gadget: Set lpm_capable

All DWC3 controllers are LPM capable. Report that in the
usb_gadget.lpm_capable for the gadget driver to properly output the
bcdUSB value in the descriptor.

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