tee: Correct inappropriate usage of TEE_SHM_DMA_BUF flag

Currently TEE_SHM_DMA_BUF flag has been inappropriately used to not
register shared memory allocated for private usage by underlying TEE
driver: OP-TEE in this case. So rather add a new flag as TEE_SHM_PRIV
that can be utilized by underlying TEE drivers for private allocation
and usage of shared memory.

With this corrected, allow tee_shm_alloc_kernel_buf() to allocate a
shared memory region without the backing of dma-buf.

Cc: stable@vger.kernel.org
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Co-developed-by: Tyler Hicks <tyhicks@linux.microsoft.com>
Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>