iommu: Move IOMMU mapping out of Ion directory

The IOMMU mapping APIs used to be tied to Ion but there
is less of a dependency these days. Move the file out of
the Ion directory and to somewhere more generic.

Change-Id: Ie38f6318c5e162820133bbff79ed3bb5918ddbc5
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>