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 <naresh.solanki@9elements.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240206105502.648255-1-naresh.solanki@9elements.com


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