dt-bindings: mailbox: Introduce Hi3660 controller binding

Introduce a binding for the Hi3660 mailbox controller, the mailbox is
used within application processor (AP), communication processor (CP),
HIFI and MCU, etc.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>