msm: scm-pas: Provide a strongly ordered buffer to the secure world

The secure world requires that the metadata-containing buffer
be strongly ordered. Use the dma_alloc_attrs API to obtain a
strongly ordered region for this purpose.

CRs-Fixed: 525863
Change-Id: I0fdfb9957aa0eb66279afa362edec075c3dff131
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>