msm: iommu: Restructure iommu domains

Each domain iova space is now partitioned via an array
of memory pools. This makes it easier to understand the
partitioning of the iova space for a given domain.

Change-Id: If83d03a4a97eb5db529cd80f1f956a3602cc4ce4
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>