device property: Add fwnode_handle_get()

fwnode_handle_get() is used to obtain a reference to a fwnode_handle
container. In this case this is OF specific struct device_node.

This complements fwnode_handle_put() which is already implemented.

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>