usb: gadget: introduce is_selfpowered for usb_gadget

Whether the gadget is selfpowerwed or not can be determined by composite
core, so we can use a common entry to indicate if the self-powered
is supported by gadget, and the related private variable at individual
udc driver can be deleted.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>