iommu-api: Add iommu_map and iommu_unmap functions

These two functions provide support for mapping and
unmapping physical addresses to io virtual addresses. The
difference to the iommu_(un)map_range() is that the new
functions take a gfp_order parameter instead of a size. This
allows the IOMMU backend implementations to detect easier if
a given range can be mapped by larger page sizes.
These new functions should replace the old ones in the long
term.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>