[CPUFREQ] implement !CONFIG_CPU_FREQ stub for cpufreq_unregister_notifier()

Callsites such as arch/powerpc/oprofile/op_model_cell.c are having to
open-code #ifdef CONFIG_CPU_FREQ only to be able to get at the full definition
of cpufreq_unregister_notifier(), because no empty stub is available for the
!CONFIG_CPU_FREQ case.  Let's provide one, to be able to remove such #ifdef's
from the rest of the kernel tree -- those will come in a subsequent patch.

Signed-off-by: Satyam Sharma <satyam@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Jones <davej@redhat.com>