doc: add documentation for mailbox framework

 Some explanations with examples of how to write to implement users
and providers of the mailbox framework.

Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
(cherry picked from commit 15320fbcec69dc3a4f217044ed848e4225397e25)
Signed-off-by: Mark Brown <broonie@kernel.org>
1 file changed