powerpc/ioda/iommu/debugfs: Generate unique debugfs entries

The iommu_table::it_index is a LIOBN which is not initialized on PowerNV
as it is not used except IOMMU debugfs where it is used for a node name.

This initializes it_index witn a unique number to avoid warnings and
have a node for every iommu_table.

This should not cause any behavioral change without CONFIG_IOMMU_DEBUGFS.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220714080800.3712998-1-aik@ozlabs.ru