powerpc/pci: Fix various pseries PCI hotplug issues

The pseries PCI hotplug code has a number of issues, ranging from
incorrect resource setup to crashes, depending on what is added,
when, whether it contains a bridge, etc etc....

This fixes a whole bunch of these, while actually simplifying the code
a bit, using more generic code in the process and factoring out common
code between adding of a PHB, a slot or a device.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>