device property: Make dev_fwnode() public

commit e44bb0cbdc88686c21e2175a990b40bf6db5d005 upstream.

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>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>