iommu: Only allow iommu_map/unmap for paging domains

Check for the new __IOMMU_DOMAIN_PAGING flag before calling
into the iommu drivers ->map and ->unmap call-backs.

Tested-by: Thierry Reding <treding@nvidia.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>