PCI: Export some PCI PM functionality

It's helpful to have some extra PCI power management functions available to
platform code, so move the declarations to an exported header.

Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>