[POWERPC] Fix up the OF functions to only do PCI stuff if PCI is actually configured

The original problem that inspired this patch was solved quite some time
ago (Turning off PCI didn't work), but this patch neatens things up a
little (I think), by putting all the PCI stuff inside a single CONFIG_PCI
block.  It also removes the OF PCI bus matching entries if CONFIG_PCI is
off.

Signed-off-by: Paul Mackerras <paulus@samba.org>