powerpc/eeh: Aux PE data for error log

The patch allows PE (struct eeh_pe) instance to have auxillary data,
whose size is configurable on basis of platform. For PowerNV, the
auxillary data will be used to cache PHB diag-data for that PE
(frozen PE or fenced PHB). In turn, we can retrieve the diag-data
at any later points.

It's useful for the case of VFIO PCI devices where the error log
should be cached, and then be retrieved by the guest at later point.
Also, it can avoid PHB diag-data overwritting if another frozen PE
reported and the previous diag-data isn't fetched by guest.

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