dt-bindings: clock: qcom,gcc-sc8280xp: document power domain

GCC clock controller is supplied by CX power domain:

  sc8280xp-crd.dtb: clock-controller@100000: Unevaluated properties are not allowed ('power-domains' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230102085909.24620-1-krzysztof.kozlowski@linaro.org