powerpc/powernv/ioda2: Introduce helpers to allocate TCE pages

This is a part of moving TCE table allocation into an iommu_ops
callback to support multiple IOMMU groups per one VFIO container.

This moves the code which allocates the actual TCE tables to helpers:
pnv_pci_ioda2_table_alloc_pages() and pnv_pci_ioda2_table_free_pages().
These do not allocate/free the iommu_table struct.

This enforces window size to be a power of two.

This should cause no behavioural change.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>