powerpc/iommu: Update the generic code to use dynamic iommu page sizes

This patch updates the generic iommu backend code to use the
it_page_shift field to determine the iommu page size instead of
using hardcoded values.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/powernv/pci.c

@@ -762,8 +762,6 @@ static struct notifier_block tce_iommu_bus_nb = {
 
 static int __init tce_iommu_bus_notifier_init(void)
 {
-	BUILD_BUG_ON(PAGE_SIZE < IOMMU_PAGE_SIZE_4K);
-
 	bus_register_notifier(&pci_bus_type, &tce_iommu_bus_nb);
 	return 0;
 }