device property: fwnode_property_read_string_array() may return -EILSEQ

fwnode_property_read_string_array() may return -EILSEQ through
of_property_read_string_array(). Document this.

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>