optee: add optee_pool_op_free_helper()

Adds a common helper function to free a tee_shm allocated using the
helper function optee_pool_op_alloc_helper().

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>