dt-bindings: iio: adc: qcom,spmi-vadc: extend example

Cleanup existing example (generic node name for spmi, use 4-space
indentation) and add example for ADCv7 copied from
Documentation/devicetree/bindings/thermal/qcom-spmi-adc-tm5.yaml.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221027143411.277980-2-krzysztof.kozlowski@linaro.org