openrisc: Make cpu_relax() invoke barrier()

Make cpu_relax() invoke barrier() to be the same as other arches.
Signed-off-by: Vladimir Murzin <murzin.v@gmail.com>
Signed-off-by: Jonas Bonn <jonas@southpole.se>

arch/openrisc/include/asm/processor.h

@@ -103,7 +103,7 @@ extern unsigned long thread_saved_pc(struct task_struct *t);
 
 #define init_stack      (init_thread_union.stack)
 
-#define cpu_relax()     do { } while (0)
+#define cpu_relax()     barrier()
 
 #endif /* __ASSEMBLY__ */
 #endif /* __ASM_OPENRISC_PROCESSOR_H */