If dr_mode is "otg" then support dual role mode of operation.
Currently this mode is only supported when an extcon handle is
present in the dwc3 device tree node. This is needed to
get the ID status events of the port.

We're using a workqueue to manage the dual-role state transitions
as the extcon notifier (dwc3_drd_notifier) is called in an atomic
context by extcon_sync() and this doesn't go well with
usb_del_gadget_udc() causing a lockdep and softirq warning.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>