USB: dwc3: get extcon device by OF graph bindings

extcon device is used to detect host/device connection. Since extcon
OF property is deprecated, alternative method should be added.
This method uses OF graph bindings to locate extcon.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>