dt-bindings: mfd: qcom,spmi-pmic: Allow RRADC as adc@ subnode

This was omitted when adding the RRADC bindings.

Fixes: 7ea58c951ab3 ("dt-bindings: iio: adc: document qcom-spmi-rradc")
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230213201337.2089970-1-konrad.dybcio@linaro.org