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