ppc64: provide default_idle, we shouldnt ever actually use this but

we need it to link.

arch/ppc64/kernel/idle.c

@@ -146,3 +146,8 @@ int cpu_idle(void)
 }
 
 #endif /* CONFIG_PPC_ISERIES */
+
+void default_idle(void)
+{
+	barrier();
+}