Documentation: dt: mailbox: Add TI Message Manager

Message Manager is a hardware block used to communicate with various
processor systems within certain Texas Instrument's Keystone
generation SoCs.

This hardware engine is used to transfer messages from various compute
entities(or processors) within the SoC. It is designed to be self
contained without needing software initialization for operation.

Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>