driver core: Add iommu_group tracking to struct device

IOMMU groups allow IOMMU drivers to represent DMA visibility
and isolation of devices.  Multiple devices may be grouped
together for the purposes of DMA.  Placing a pointer on
struct device enable easy access for things like streaming
DMA programming and drivers like VFIO.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>