arm64: dts: qcom: sc8180x: Add missing 'cache-unified' to L3

Add the missing property to fix the dt checker warning:

qcom/sc8180x-primus.dtb: l3-cache: 'cache-unified' is a required property

Fixes: 8575f197 ("arm64: dts: qcom: Introduce the SC8180x platform")
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230626-topic-bindingsfixups-v1-7-254ae8642e69@linaro.org


Signed-off-by: Bjorn Andersson <andersson@kernel.org>