tee: optee: Move pool_op helper functions

Move the pool alloc and free helper functions from the OP-TEE driver to
the TEE subsystem, since these could be reused in other TEE drivers.
This patch is not supposed to change behavior, it's only reorganizing
the code.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Suggested-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Balint Dobszay <balint.dobszay@arm.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>