powerpc/powernv: Debugfs directory for PHB

The patch creates one debugfs directory ("powerpc/PCIxxxx") for
each PHB so that we can hook EEH error injection debugfs entry
there in proceeding patch.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>