arm64: dts: qcom: sc8180x: drop incorrect cell-index from SPMI

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

  sc8180x-primus.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>
Link: https://lore.kernel.org/r/20230827122842.63741-2-krzysztof.kozlowski@linaro.orgSigned-off-by: Bjorn Andersson <andersson@kernel.org>

arch/arm64/boot/dts/qcom/sc8180x.dtsi

@@ -3261,7 +3261,6 @@ spmi_bus: spmi@c440000 {
 			#size-cells = <0>;
 			interrupt-controller;
 			#interrupt-cells = <4>;
-			cell-index = <0>;
 		};
 
 		apps_smmu: iommu@15000000 {