usb: dwc3: omap: stop using nop-usb-xceiv

Now that we have drivers for omap-usb2 phy and omap-usb3 phy, stop
using nop-usb-xceiv.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>