gpu: ion: Add cache operations to IOMMU heap

The IOMMU heap must support cache operations so that clients
can do cached allocations from the heap.

Add function to perform cache operations on memory
allocated from IOMMU heap.

Change-Id: I49a84dca613b7c209dde03b67b2fc1b2c9d844dd
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>