Just like in cpu_halt(), use cpu_shutdown() rather than ofw_exit()

directly in cpu_reset() in order to idle the APs before exiting
the kernel and letting the BSP enter the firmware so that processes
like init(8) which still might be running on an AP at that point
don't cause a panic there when it crashes due to the fact it no
longer can be supported by the kernel.

MFC after:	3 days

sys/sparc64/sparc64/vm_machdep.c

@@ -342,7 +342,7 @@ cpu_reset(void)
 		bspec[sizeof(bspec) - 1] = '\0';
 	}
 
-	ofw_exit(&args);
+	cpu_shutdown(&args);
 }
 
 /*