[POWERPC] Always build setup-bus.c on powerpc

The common arch/powerpc code calls in to functions in setup-bus.c
so some builds of ppc32 would fail.

Note, ppc32 usage of setup-irq.c is limited to arch/ppc and should be
removed when arch/ppc goes away.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>