usb: dwc3: add in_lpm flag to struct dwc3

This change provides the gadget with awareness regarding
the vendor-specific's low-power-mode status. This is done
in order to enable proper decision making regarding cases
where the gadget would potentially attempt to interface the
hardware when it's in lpm.

Change-Id: I1508fbddd13228fbb7d6d53d91d98a0acaaf3fd7
Signed-off-by: Bar Weiner <bweiner@codeaurora.org>