tee: optee: Fix missing devices unregister during optee_remove

When OP-TEE driver is built as a module, OP-TEE client devices
registered on TEE bus during probe should be unregistered during
optee_remove. So implement optee_unregister_devices() accordingly.

Fixes: c3fa24af ("tee: optee: add TEE bus device enumeration support")
Reported-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>