dt-bindings: thermal: Use generic ADC node name in examples

Update the examples to reflect a future requirement for the generic
`channel` node name on ADC channel nodes, while conveying the board name
of the channel in a label instead.

Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230410202917.247666-4-marijn.suijten@somainline.org