frv: Define cpu_relax_lowlatency()

3a6bfbc9 "(arch,locking: Ciao arch_mutex_cpu_relax()") broke
building the frv arch.  Fixes errors such as:

  kernel/locking/mcs_spinlock.h:87:2: error: implicit declaration of function 'cpu_relax_lowlatency'
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Compile-tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/frv/include/asm/processor.h

@@ -129,7 +129,8 @@ unsigned long get_wchan(struct task_struct *p);
 #define	KSTK_EIP(tsk)	((tsk)->thread.frame0->pc)
 #define	KSTK_ESP(tsk)	((tsk)->thread.frame0->sp)
 
-#define cpu_relax()    barrier()
+#define cpu_relax() barrier()
+#define cpu_relax_lowlatency() cpu_relax()
 
 /* data cache prefetch */
 #define ARCH_HAS_PREFETCH