PCI: generic: Add pci_host_common_probe(), based on gen_pci_probe()

Factor gen_pci_probe(), moving most of it into pci_host_common_probe()
where it can be shared with other drivers that have slightly different
config accessors.  No functional change intended.

[bhelgaas: split into separate patch, changelog]
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Will Deacon <will.deacon@arm.com>