By using struct led_init_data when registering we do not need to parse
`label` DT property nor `linux,default-trigger` property.
A small refactor was also done:
- with using devm_led_classdev_register_ext the driver remove method is
not needed
- since only one child node is allowed for this driver, use
device_get_next_child_node instead of device_for_each_child_node
Previously if the `label` DT property was not present, the code composed
name for the LED in the form
"parent_name::"
For backwards compatibility we therefore set
init_data->default_label = ":";
so that the LED will not get a different name if `label` property is not
present.