As defined in PAPR 2.4, Partitionable Endpoint (PE) is an I/O subtree
that can be treated as a unit for the purposes of partitioning and error
recovery. Therefore, eeh core should be aware of PE. With eeh_pe struct,
we can support PE explicitly. Further more, it makes all the stuff much
more data centralized. Another important reason is for eeh core to support
multiple platforms. Some of them like pSeries figures out PEs through
OF nodes while others like powernv have to do that through PCI bus/device
tree. With explicit PE support, eeh core will be implemented based on
the centrialized data and platform dependent implementations figure it
out by their feasible ways.

When the struct is designed, following factors are taken in account:
  * Reflecting the relationships of PEs. PE might have parent
    as well children.
  * Reflecting the association of PE and (eeh) devices.
  * PEs have PHB boundary.
  * PE should have unique address assigned in the corresponding
    PHB domain.
Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>