Move definitions for generic PCI host controller driver structures to a
separate header file so we can share them with other drivers.  No
functional change intended.

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