tee: add kernel internal client interface

Adds a kernel internal TEE client interface to be used by other drivers.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Tested-by: Sumit Garg <sumit.garg@linaro.org>
Tested-by: Zeng Tao <prime.zeng@hisilicon.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>