MPM provides a single genpd. Allow #power-domain-cells = <0>.

Fixes: 54fc9851 ("dt-bindings: interrupt-controller: Add Qualcomm MPM support")
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Link: https://lore.kernel.org/r/20231129-topic-mpmbindingspd-v2-1-acbe909ceee1@linaro.orgSigned-off-by: Rob Herring <robh@kernel.org>