tee: allow a driver to allocate a tee_device without a pool

tee: allow a driver to allocate a tee_device without a pool

A TEE driver doesn't always need to provide a pool if it doesn't
support memory sharing ioctls and can allocate memory for TEE
messages in another way. Although this is mentioned in the
documentation for tee_device_alloc(), it is not handled correctly.

Reviewed-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
Signed-off-by: Amirreza Zarrabi <amirreza.zarrabi@oss.qualcomm.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>