mailbox: Introduce TI message manager driver

Support for TI Message Manager Module. This hardware block manages a
bunch of hardware queues meant for communication between processor
entities.

Clients sitting on top of this would manage the required protocol
for communicating with the counterpart entities.

For more details on TI Message Manager hardware block, see documentation
that will is available here: http://www.ti.com/lit/ug/spruhy8/spruhy8.pdf
Chapter 8.1(Message Manager)

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