mailbox: Add device-managed registration functions

Add device-managed equivalents of the mbox_controller_register() and
mbox_controller_unregister() functions that can be used to have the
devres infrastructure automatically unregister mailbox controllers on
driver probe failure or driver removal. This can help remove a lot of
boiler plate code from drivers.

Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>