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 <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Reviewed-by: Quentin Schulz <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

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

···
    8 | batt_v
    9 | batt_chrg_i
   10 | batt_dischrg_i
+  11 | ts_v
 
   AXP22x
   ------
···
    1 | batt_v
    2 | batt_chrg_i
    3 | batt_dischrg_i
+   4 | ts_v
 
   AXP813
   ------
···
    2 | batt_v
    3 | batt_chrg_i
    4 | batt_dischrg_i
+   5 | ts_v
 
 
 properties: