powerpc/eeh: pseries platform EEH initialization

The platform specific EEH operations have been abstracted by
struct eeh_ops. The individual platroms, including pSeries, needs
doing necessary initialization before the platform dependent EEH
operations work properly.

The patch is addressing that and do necessary platform initialization
for pSeries platform. More specificly, it will figure out the tokens
of EEH related RTAS calls.

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