tee: tee_shm_op_mmap(): use TEE_SHM_USER_MAPPED

tee_shm_op_mmap() uses the TEE_SHM_USER_MAPPED flag instead of the
TEE_SHM_REGISTER flag to tell if a shared memory object is originating
from registered user space memory.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>