iommu: Add apply_dm_region call-back to iommu-ops

This new call-back will be used by the iommu driver to do
reserve the given dm_region in its iova space before the
mapping is created.

The call-back is temporary until the dma-ops implementation
is part of the common iommu code.

Signed-off-by: Joerg Roedel <jroedel@suse.de>