powerpc/eeh: Keep PE during hotplug

When we do normal hotplug, the PE (shadow EEH structure) shouldn't be
kept around.

However, we need to keep it if the hotplug an artifial one caused by
EEH errors recovery.

Since we remove EEH device through the PCI hook pcibios_release_device(),
the flag "purge_pe" passed to various functions is meaningless. So the patch
removes the meaningless flag and introduce new flag "EEH_PE_KEEP"
to save the PE while doing hotplug during EEH error recovery.

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