msm: secure_buffer: warn if we get a non-1MB-aligned chunk

We only support securing memory in 1MB chunks. If we get an sg table
with any entries that are of any other size, WARN and bail.

Change-Id: Icd96984f03dafd246da51e879cd91c77ae986724
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>