powerpc/eeh: Selectively enable IO for error log

According to the experiment I did, PCI config access is blocked
on P7IOC frozen PE by hardware, but PHB3 doesn't do that. That
means we always get 0xFF's while dumping PCI config space of the
frozen PE on P7IOC. We don't have the problem on PHB3. So we have
to enable I/O prioir to collecting error log. Otherwise, meaningless
0xFF's are always returned.

The patch fixes it by EEH flag (EEH_ENABLE_IO_FOR_LOG), which is
selectively set to indicate the case for: P7IOC on PowerNV platform,
pSeries platform.

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