staging: android: ion: Support iommu mappings with one segment

Ensure that ION supports iommu mappings with one segment.

Unfortunately with partial cache maintenance it is not possible to support
mappings with multiple segments as these mappings could include IOVA
padding and this makes it very difficult to calculate the IOVA for an
offset in an SG entry.
This limitation should not be a problem as we expect ION clients to
only use mappings with one segment.

Change-Id: I3cb88398cd41a68faa77fb64ddcfdee9d9ecdfbd
Signed-off-by: Liam Mark <lmark@codeaurora.org>