PPC: E500: Bump CPU count to 15

Now that we have everything in place, make the machine description
aware of the fact that we can now handle 15 virtual CPUs!
Signed-off-by: NAlexander Graf <agraf@suse.de>

---

v1 -> v2:

  - Max cpus is 15 because of MPIC

hw/ppce500_mpc8544ds.c

@@ -396,6 +396,7 @@ static QEMUMachine mpc8544ds_machine = {
     .name = "mpc8544ds",
     .desc = "mpc8544ds",
     .init = mpc8544ds_init,
+    .max_cpus = 15,
 };
 
 static void mpc8544ds_machine_init(void)