dt-bindings: mailbox: Add Xilinx IPI Mailbox

Xilinx ZynqMP IPI(Inter Processor Interrupt) is a hardware block
in ZynqMP SoC used for the communication between various processor
systems.

Signed-off-by: Wendy Liang <wendy.liang@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>