Add VADC channels of PM8280_{1/2} PMICs for measuring the on-chip die
temperature and external thermistors connected to the AMUX pins.

The measurements are collected by the primary PMIC PMK8280 from the
secondary PMICs PM8280_{1/2} and exposed over the PMK8280's VADC channels.
Reviewed-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221103095810.64606-10-manivannan.sadhasivam@linaro.org