USB driver in Chopstx

Full speed device is assumed.  Bus powered system is assumed.

API-wise, self powered system is not supported (yet).  To support self
powered system, we need to define the hardware interface detecting
VBUS voltage.  Only after detecting VBUS power, we can enable USB
driver (D+/D- lines).  For self powered system, driving D+/D- lines
(by fixed pull-up resistor) violates the USB specification.