Add mailbox support required in IPQ8074 SoCs.

Signed-off-by: Gokul Sriram Palanisamy <gokulsri@codeaurora.org>
Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>