PPC: Make MPC8544DS obey -cpu switch

The MPC8544DS board emulation code ignored the user defined -cpu switch.
This patch enables it to only provide a sane default, not force an e500v2
CPU inside.

Signed-off-by: Alexander Graf <agraf@suse.de>
diff --git a/hw/ppce500_mpc8544ds.c b/hw/ppce500_mpc8544ds.c
index e111dda..1b8a1c4 100644
--- a/hw/ppce500_mpc8544ds.c
+++ b/hw/ppce500_mpc8544ds.c
@@ -178,7 +178,11 @@
     qemu_irq *irqs, *mpic, *pci_irqs;
 
     /* Setup CPU */
-    env = cpu_ppc_init("e500v2_v30");
+    if (cpu_model == NULL) {
+        cpu_model = "e500v2_v30";
+    }
+
+    env = cpu_ppc_init(cpu_model);
     if (!env) {
         fprintf(stderr, "Unable to initialize CPU!\n");
         exit(1);