[POWERPC] pci32: Add flags modifying the PCI code behaviour

This adds to the 32 bits PCI code some flags, replacing the old
pci_assign_all_busses global, that allow us to control various
aspects of the PCI probing, such as whether to re-assign all
resources or not, or to not try to assign anything at all.

This also adds the flag x86 already has to avoid ISA alignment
on bridges that don't have ISA forwarding enabled (no legacy
devices on the top level bus) and sets it for PowerMacs.

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