powernv: Fix VFIO support with PHB3

I have recently found out that no iommu_groups could be found under
/sys/ on a P8. That prevents PCI passthrough from working.

During my investigation, I found out there seems to be a missing
iommu_register_group for PHB3. The following patch seems to fix the
problem. After applying it, I see iommu_groups under
/sys/kernel/iommu_groups/, and can also bind vfio-pci to an adapter,
which gives me a device at /dev/vfio/.
Signed-off-by: NThadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/powernv/pci-ioda.c

@@ -720,6 +720,7 @@ static void pnv_pci_ioda2_setup_dma_pe(struct pnv_phb *phb,
 		tbl->it_type = TCE_PCI_SWINV_CREATE | TCE_PCI_SWINV_FREE;
 	}
 	iommu_init_table(tbl, phb->hose->node);
+	iommu_register_group(tbl, pci_domain_nr(pe->pbus), pe->pe_number);
 
 	if (pe->pdev)
 		set_iommu_table_base(&pe->pdev->dev, tbl);