frv: Define cpu_relax_lowlatency()

3a6bfbc91df0 "(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>