software node: Add software_node_get_reference_args()

This makes it possible to support drivers that use
fwnode_property_get_reference_args() function.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>