iio: provide of_iio_channel_get_by_name() and devm_ version it

There might be cases when the IIO channel is attached to the device
subnode instead of being attached to the main device node. Allow drivers
to query IIO channels by using device tree nodes.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20201204025509.1075506-8-dmitry.baryshkov@linaro.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>