[POWERPC] Fixup powermac enable device hook

Powermac's use of the pcibios_enable_device_hook() got slightly
broken by the recent PCI merge in that it won't be called for
the "initial" case of assigning resources to a previously
unassigned device.  This was an abuse of that hook anyway, so
instead we now use a header quirk.

While at it, we move a #ifdef CONFIG_PPC32 to enclose more code
that is only ever used on 32 bits.

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