arm64: dts: qcom: drop incorrect cell-index from SPMI

The SPMI controller (PMIC Arbiter)) does not use nor allow 'cell-index'
property:

  sm8150-microsoft-surface-duo.dtb: spmi@c440000: Unevaluated properties are not allowed ('cell-index' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230308125906.236885-1-krzysztof.kozlowski@linaro.org