It's know that Intel's SoCs' dwc3 integration is
peripheral-only since Intel implements its own
portmux for role-swapping. In order to prevent dwc3
from ever registering and XHCI platform_device,
let's just set dr-mode to peripheral-only on Intel
SoCs.
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>