usb: dwc3: pci: Intel Merrifield can be host

On Intel Edison board the OTG function is enabled, thus,
USB can switch to the host mode.

Allow that by changing dr_mode property to "otg" for Intel Merrifield.

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