dt-bindings: mailbox: add compatible for the IPQ6018 SoC

Add the mailbox compatible for the IPQ6018 SoC.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kathiravan T <kathirav@codeaurora.org>
Link: https://lore.kernel.org/r/1597648720-13649-2-git-send-email-kathirav@codeaurora.org


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