dt-bindings: iio: adc: Require generic `channel` name for channel nodes

dt-bindings: iio: adc: Require generic `channel` name for channel nodes

As discussed in [1] it is more convenient to use a generic `channel`
node name for ADC channels while storing a friendly - board-specific
instead of PMIC-specific - name in the label, if/when desired to
overwrite the channel description already contained (but previously
unused) in the driver [2].

The same `channel` node name pattern has also been set in
iio/adc/adc.yaml, but this generic binding is not inherited as base for
qcom,spmi-vadc bindings due to not having any other generic elements in
common, besides the node name rule and reg property.

Replace the .* name pattern with the `channel` literal, but leave the
label property optional for bindings to choose to fall back a channel
label hardcoded in the driver [2] instead.

[1]: https://lore.kernel.org/linux-arm-msm/20221106193018.270106-1-marijn.suijten@somainline.org/T/#u
[2]: https://lore.kernel.org/linux-arm-msm/20230116220909.196926-4-marijn.suijten@somainline.org/

Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230410202917.247666-6-marijn.suijten@somainline.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>