arm64: dts: qcom: Use labels with generic node names for ADC channels

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].  Follow up on the dt-bindings' `channel` node
name requirement, and instead provide this (sometimes per-board) channel
description through a label property.

Also remove all the unused label references (not to be confused with
label properties) from pm660, pmp8074 and pms405.

[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/

Acked-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20230730-generic-adc-channels-v5-2-e6c69bda8034@somainline.org
Signed-off-by: Bjorn Andersson <andersson@kernel.org>