This gets rid of some of the special cases in the bus scanning function
by splitting hierarchy enumeration and configuration and the actual
device registration into 2 passes.

This ensures that the PCI hierarchy below a root port is completely
set up before any device driver is probed.

This simplifies the code and makes it less error prone, while moving
the PCI address space layout closer to the one used by Linux.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>