dt-bindings: iio: adc: document TS voltage in AXP PMICs

Most AXPxxx-based reference designs place a 10k NTC thermistor on a
TS pin. axp20x IIO driver now report the voltage of this pin via
additional IIO channel. Add new "ts_v" channel to the channel description.

Signed-off-by: Evgeny Boger <boger@wirenboard.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Quentin Schulz <foss+kernel@0leil.net>
Link: https://lore.kernel.org/r/20211118141233.247907-3-boger@wirenboard.com


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