Add the missing property to fix the dt checker warning:

qcom/apq8039-t2.dtb: l2-cache: 'cache-unified' is a required property

Fixes: 61550c6c ("arm64: dts: qcom: Add msm8939 SoC")
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Link: https://lore.kernel.org/r/20230627-topic-more_bindings-v1-3-6b4b6cd081e5@linaro.orgSigned-off-by: Bjorn Andersson <andersson@kernel.org>