dt-bindings: clock: qcom,gcc-sc7180: document CX power domain

The GCC clock controller needs CX power domain, at least according to
DTS:

  sc7180-trogdor-pompom-r3.dtb: clock-controller@100000: Unevaluated properties are not allowed ('power-domains' was unexpected)

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230516105241.30091-2-krzysztof.kozlowski@linaro.org