PCI: Add pcibios_pm_ops for optional arch-specific hibernate functionality

Platforms may want to provide architecture-specific functionality when
a PCI device is doing a hibernate transition.  Add a weak symbol
pcibios_pm_ops that architectures can override to do so.

[bhelgaas: fold in return value checks from v2 patch]
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>