iio: max31856: add missing of_node and parent references to iio_dev

Adding missing indio_dev->dev.of_node references so that, in case multiple
max31856 are present, users can get some clues to being able to distinguish
each of them. While at it, add also the missing parent reference.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>