dt-bindings: mailbox: Introduce Qualcomm APCS global binding

Introduce a binding for the Qualcomm APCS global block, exposing a
mailbox for invoking interrupts on remote processors in the system.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>