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

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

  qcom-sdx55-mtp.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/20230827122842.63741-1-krzysztof.kozlowski@linaro.orgSigned-off-by: Bjorn Andersson <andersson@kernel.org>

arch/arm/boot/dts/qcom/qcom-sdx55.dtsi

@@ -638,7 +638,6 @@ spmi_bus: spmi@c440000 {
 			#size-cells = <0>;
 			interrupt-controller;
 			#interrupt-cells = <4>;
-			cell-index = <0>;
 		};
 
 		tlmm: pinctrl@f100000 {

arch/arm/boot/dts/qcom/qcom-sdx65.dtsi

@@ -544,7 +544,6 @@ spmi_bus: qcom,spmi@c440000 {
 			#interrupt-cells = <4>;
 			#address-cells = <2>;
 			#size-cells = <0>;
-			cell-index = <0>;
 			qcom,channel = <0>;
 			qcom,ee = <0>;
 		};