powerpc/dart: iommu table cleanup

No need to set the device tree device_node pci node iommu pointer, its
only used for dlpar remove.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>