PCI: generic: Move structure definitions to separate header file

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: 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>