device property: Make dev_fwnode() public

The function to obtain a fwnode related to a struct device is useful for
drivers that use the fwnode property API: it allows not being aware of the
underlying firmware implementation.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>