usb: dwc3: pci: Enable extcon driver for Intel Merrifield

Intel Merrifield provides a DR support via PMIC which has its own
extcon driver.

Add a property string to link to that driver.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>