usb: dwc3: drd: Add support for DR detection through extcon

Allow extcon device, found by name, to provide DR status for USB.
This is needed, for example, in case of Intel Merrifield platform,
where the Intel Basin Cove PMIC provides an extcon device to communicate
the detected role.

Note, that the "linux,extcon-name" property name is only for kernel
internal use by X86/ACPI platform code and as such is not documented
in the device tree bindings.

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