MAINTAINERS: Update PPC contacts for PCI core error handling

The original PCI error recovery functionality was for the powerpc-specific
IBM EEH feature.  PCIe subsequently added some similar features, including
AER and DPC, that can be used on any architecture.

We want the generic PCI core error handling support to work with all of
these features.  Driver error recovery callbacks should be independent of
which feature the platform provides.

Add the generic PCI core error recovery files to the powerpc EEH
MAINTAINERS entry so the powerpc folks will be copied on changes to the
generic PCI error handling strategy.

Add Sam and Oliver as maintainers for this area.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Russell Currey <ruscur@russell.cc>
Acked-by: Sam Bobroff <sbobroff@linux.ibm.com>