Adds the two new functions tee_shm_register_user_buf() and
tee_shm_register_kernel_buf() which should be used instead of the old
tee_shm_register().

This avoids having the caller supplying the flags parameter which
exposes a bit more than desired of the internals of the TEE subsystem.
Reviewed-by: NSumit Garg <sumit.garg@linaro.org>
Signed-off-by: NJens Wiklander <jens.wiklander@linaro.org>