powerpc/pseries: Rivet CONFIG_EEH for pSeries platform

Recently, Ryan Wang tried to compile PPC pSeries platform without
CONFIG_EEH and eventually run into errors. Nishanth Aravamudan
helped to narrow down the root cause. Actually, the pSeries platform
depends on CONFIG_EEH heavily and that won't work properly without
EEH support.

According to Ben's suggestion, the patch make CONFIG_EEH invisible
and keep it as always selected on pSeries platform.

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