usb: dwc3: omap: add nop transceiver support

We will be adding support for transceivers on
dwc3 driver but not all boards have controllable
transceivers.

For those which don't provide controllable transceivers
we will register nop transceivers.

Note that once OMAP's transceiver drivers reach mainline,
this glue layer will change accordingly.

Signed-off-by: Felipe Balbi <balbi@ti.com>