powerpc/eeh: Create eeh_dev from pci_dn instead of device_node

The patch adds function traverse_pci_dn(), which is similar to
traverse_pci_devices() except it takes pci_dn, not device_node
as parameter. The pci_dev.c has been reworked to create eeh_dev
from pci_dn, instead of device_node.

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