iommu: Add sysfs support for IOMMUs

IOMMUs currently have no common representation to userspace, most
seem to have no representation at all aside from a few printks
on bootup.  There are however features of IOMMUs that are useful
to know about.  For instance the IOMMU might support superpages,
making use of processor large/huge pages more important in a device
assignment scenario.  It's also useful to create cross links between
devices and IOMMU hardware units, so that users might be able to
load balance their devices to avoid thrashing a single hardware unit.

This patch adds a device create and destroy interface as well as
device linking, making it very lightweight for an IOMMU driver to add
basic support.  IOMMU drivers can provide additional attributes
automatically by using an attribute_group.

The attributes exposed are expected to be relatively device specific,
the means to retrieve them certainly are, so there are currently no
common attributes for the new class created here.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>