firmware: arm_ffa: Unregister the FF-A devices when cleaning up the partitions

Both the FF-A core and the bus were in a single module before the
commit 18c250bd ("firmware: arm_ffa: Split bus and driver into distinct modules").

The arm_ffa_bus_exit() takes care of unregistering all the FF-A devices.
Now that there are 2 distinct modules, if the core driver is unloaded and
reloaded, it will end up adding duplicate FF-A devices as the previously
registered devices weren't unregistered when we cleaned up the modules.

Fix the same by unregistering all the FF-A devices on the FF-A bus during
the cleaning up of the partitions and hence the cleanup of the module.

Fixes: 18c250bd ("firmware: arm_ffa: Split bus and driver into distinct modules")
Tested-by: Viresh Kumar <viresh.kumar@linaro.org>
Message-Id: <20250217-ffa_updates-v3-8-bd1d9de615e7@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>