tee: replace tee_shm_alloc()

tee_shm_alloc() is replaced by three new functions,

tee_shm_alloc_user_buf() - for user mode allocations, replacing passing
the flags TEE_SHM_MAPPED | TEE_SHM_DMA_BUF

tee_shm_alloc_kernel_buf() - for kernel mode allocations, slightly
optimized compared to using the flags TEE_SHM_MAPPED | TEE_SHM_DMA_BUF.

tee_shm_alloc_priv_buf() - primarily for TEE driver internal use.

This also makes the interface easier to use as we can get rid of the
somewhat hard to use flags parameter.

The TEE subsystem and the TEE drivers are updated to use the new
functions instead.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>