mailbox: Introduce framework for mailbox

mailbox: Introduce framework for mailbox

Introduce common framework for client/protocol drivers and
controller drivers of Inter-Processor-Communication (IPC).

Client driver developers should have a look at
 include/linux/mailbox_client.h to understand the part of
the API exposed to client drivers.
Similarly controller driver developers should have a look
at include/linux/mailbox_controller.h

Reviewed-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
(cherry picked from commit 2b6d83e2b8b7de82331a6a1dcd64b51020a6031c)
Signed-off-by: Mark Brown <broonie@kernel.org>