arm64: dts: qcom: ipq8074: add compatible fallback to mailbox

IPQ8074 mailbox is compatible with IPQ6018.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230322174148.810938-4-krzysztof.kozlowski@linaro.org