driver, pci: add functions to add/del devices to/from root-cell

driver, pci: add functions to add/del devices to/from root-cell

This patch introduces functions to add or remove PCI devices in the root
cell. The functions are not called yet, the patch prepares for virtual
PCI devices.
The original idea was to remove PCI devices from Linux on cell create
and re-add them on cell destroy. But in this case Linux could reprogram
the BARs so we ignore regular PCI devices for now.

Signed-off-by: Henning Schild <henning.schild@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>