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

Documentation/devicetree/bindings/iio/adc/x-powers,axp209-adc.yaml

@@ -27,6 +27,7 @@ description: |
    8 | batt_v
    9 | batt_chrg_i
   10 | batt_dischrg_i
+  11 | ts_v
 
   AXP22x
   ------
@@ -34,6 +35,7 @@ description: |
    1 | batt_v
    2 | batt_chrg_i
    3 | batt_dischrg_i
+   4 | ts_v
 
   AXP813
   ------
@@ -42,6 +44,7 @@ description: |
    2 | batt_v
    3 | batt_chrg_i
    4 | batt_dischrg_i
+   5 | ts_v
 
 
 properties: