powerpc/pseries: Don't try to register pseries cpu hotplug on non-pseries

This results in oddball messages at boot on other platforms telling us
that CPU hotplug isn't supported even when it is.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/pseries/hotplug-cpu.c

@@ -420,4 +420,4 @@ static int __init pseries_cpu_hotplug_init(void)
 
 	return 0;
 }
-arch_initcall(pseries_cpu_hotplug_init);
+machine_arch_initcall(pseries, pseries_cpu_hotplug_init);