Synchronize access to shm or shared memory buffer list to prevent
race conditions due to concurrent updates to shared shm list by
multiple threads.

Fixes: 757cc3e9 ("tee: add AMD-TEE driver")
Reviewed-by: NDevaraj Rangasamy <Devaraj.Rangasamy@amd.com>
Signed-off-by: NRijo Thomas <Rijo-john.Thomas@amd.com>
Signed-off-by: NJens Wiklander <jens.wiklander@linaro.org>