dt-bindings: iio: afe: voltage-divider: Add io-channel-cells

Enable the voltage divider to both receive and provide measurement
services by adding #io-channel-cells.

This is especially valuable in scenarios where an ADC has an analog
frontend, like a voltage divider, and obtaining its raw value isn't
interesting. It is desired to get the real voltage before the voltage
divider.

Signed-off-by: Naresh Solanki <[e-mail]>
Reviewed-by: Conor Dooley <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/devicetree/bindings/iio/afe/voltage-divider.yaml

···
     description: |
       Channel node of a voltage io-channel.
 
+  '#io-channel-cells':
+    description:
+      In addition to consuming the measurement services of a voltage
+      output channel, the voltage divider can act as a provider of
+      measurement services to other devices. This is particularly
+      useful in scenarios wherein an ADC has an analog frontend,
+      such as a voltage divider, and then consuming its raw value
+      isn't interesting. In this case, the voltage before the divider
+      is desired.
+    const: 1
+
   output-ohms:
     description:
       Resistance Rout over which the output voltage is measured. See full-ohms.