powerpc/eeh: Remove eeh device from OF node

Originally, the PCI sensitive OF node is tracing the eeh device
through struct device_node->edev. However, it was regarded as
bad idea.

The patch removes struct device_node->edev and uses PCI_DN to
trace the corresponding eeh device according to BenH's comments.

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