firmware: arm_scpi: make freeing mbox channels device-managed

Make freeing the mbox channels device-managed, thus further simplifying
scpi_remove and and one further step to get rid of scpi_remove.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>