powerpc/powernv: Move PNV_EEH_STATE_ENABLED around

The flag PNV_EEH_STATE_ENABLED is put into pnv_phb::eeh_state,
which is protected by CONFIG_EEH. We needn't that. Instead, we
can have pnv_phb::flags and maintain all flags there, which is
the purpose of the patch. The patch also renames PNV_EEH_STATE_ENABLED
to PNV_PHB_FLAG_EEH.

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