omap iommu: Introduce iteration macro for iotlb entry scan

There are some places to scan iotlb entries. This iteration macro
could make these code a bit simpler with proceeding iotlb entries
transparently.

Signed-off-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>
Tested-by: Hari Kanigeri <h-kanigeri2@ti.com>