PCI: generic: Expose pci_host_common_probe() for use by other drivers

Move pci_host_common_probe() and associated functions to pci-host-common.c,
where it can be shared with other drivers.  Make it public (not static)
and update Kconfig and Makefile to build it.  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>