EEH init cleanup

[Oliver O'Halloran]

This series reworks how EEH devices are initialised. This mainly affects
pseries since it moves the "early" EEH probe out of shared code and into
pseries platform code. The goal here is to make the platform
dependencies more explicit and to allow PowerNV to implement its own
pci_dev <-> eeh_dev mapping that doesn't require a PCI_DN.