iio: core: introduce trough info element for minimum values

The IIO_CHAN_INFO_PEAK info element is used for maximum values and
currently there is no equivalent for minimum values. Instead of
overloading the existing peak info element, a new info element can
be added.

In principle there is no need to add a _TROUGH_SCALE element as the
scale will be the same as the one required for INFO_PEAK, which in
turn is sometimes omitted if a single scale for peaks and raw values
is required.

Add an IIO_CHAN_INFO_TROUGH info element for minimum values.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://lore.kernel.org/r/20231211122747.9723-1-579lpy@gmail.com


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