powerpc/iommu/debug: Add debugfs entries for IOMMU tables

This adds a folder per LIOBN under /sys/kernel/debug/iommu with IOMMU
table parameters.

This is enabled by 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/20210113102014.124452-1-aik@ozlabs.ru