optee: allocate shared memory with alloc_pages_exact()

Allocate memory to share with the secure world using alloc_pages_exact()
instead of alloc_pages() for more efficient memory usage.

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