USB: dwc3: gadget: Implement gadget_vbus_draw() API

Since gadget_vbus_draw() is not implemented, not notifying
charger driver when composite layer calls this api to start/stop
charging. Hence implement gadget_vbus_draw() api to delegate
to OTG driver as OTG driver will take care of notifying charger
driver about charging.

Change-Id: I800215ddf2b18ca390b7632fb92c0e446c2f73dc
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>