iio: core: Add optional symbolic label to a device channel

If a label is defined in the device tree for this channel add that
to the channel specific attributes. This is useful for userspace to
be able to identify an individual channel.

Signed-off-by: Cristian Pop <cristian.pop@analog.com>
Link: https://lore.kernel.org/r/20200928090959.88842-1-cristian.pop@analog.com


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