powerpc/eeh: Make EEH operations based on PE

Originally, all the EEH operations were implemented based on OF node.

Actually, it explicitly breaks the rules that the operation target
is PE instead of device. Therefore, the patch makes all the operations
based on PE instead of device.

Unfortunately, the backend for config space has to be kept as original
because it doesn't depend on PE.

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