powerpc/iommu: Add it_page_shift field to determine iommu page size

This patch adds a it_page_shift field to struct iommu_table and
initiliases it to 4K for all platforms.

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