Adds a new function tee_shm_alloc_user_buf() for user mode allocations,
replacing passing the flags TEE_SHM_MAPPED | TEE_SHM_DMA_BUF to
tee_shm_alloc().
Reviewed-by: NSumit Garg <sumit.garg@linaro.org>
Signed-off-by: NJens Wiklander <jens.wiklander@linaro.org>