powerpc: Add iommu domain pointer to device archdata

Add an iommu domain pointer to device (powerpc) archdata.  Devices
are attached to iommu domains and this pointer provides a mechanism
to correlate between a device and the associated iommu domain.  This
field is set when a device is attached to a domain.

Signed-off-by: Varun Sethi <Varun.Sethi@freescale.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Joerg Roedel <joro@8bytes.org>