arm64: dts: qcom: pm8998: Add ADC Thermal Monitor node

Add a node for the ADC Thermal Monitor found in the PM8998 PMIC. This is
used to connect thermal zones with ADC channels.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Vinod Koul <vkoul@kernel.org>
Link: https://lore.kernel.org/r/20211005032531.2251928-4-bjorn.andersson@linaro.org