powernv/pci: Add a debugfs entry to dump PHB's IODA PE state

Add a debugfs entry to dump the state of the active IODA PEs. The IODA
PE state reflects how the PHB's internal concept of a PE is
configured. This is separate to the EEH PE state and is managed power
the PowerNV PCI backend rather than the EEH core.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
[mpe: Use DEFINE_DEBUGFS_ATTRIBUTE]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190912052945.12589-3-oohall@gmail.com