usb: dwc2: gadget: add vbus_draw support

This callback informs the driver about the total amount of current it
is allowed to draw. Share this information with the phy so that
current limits can be set for charging for example.

Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>