powerpc/powernv/ioda2: Fix calculation for memory allocated for TCE table

The existing code stores the amount of memory allocated for a TCE table.
At the moment it uses @offset which is a virtual offset in the TCE table
which is only correct for a one level tables and it does not include
memory allocated for intermediate levels. When multilevel TCE table is
requested, WARN_ON in tce_iommu_create_table() prints a warning.

This adds an additional counter to pnv_pci_ioda2_table_do_alloc_pages()
to count actually allocated memory.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>