usb: dwc2: Initialize the USB core for peripheral mode

Initialize the USB driver to peripheral mode when a B-Device connector
is attached.

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>