Make freeing the mbox channels device-managed, thus further simplifying
scpi_remove and and one further step to get rid of scpi_remove.
Tested-by: NKevin Hilman <khilman@baylibre.com>
Signed-off-by: NHeiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: NSudeep Holla <sudeep.holla@arm.com>