cpufreq: Add dummy cpufreq_cpu_get/put for CONFIG_CPU_FREQ=n

The drm/i915 driver wants to adjust it's own power policies using the
cpu policies as a guideline (we can implicitly boost the cpus through
the gpus on some platforms). To avoid a dreaded select (since a
depends will leave users wondering where where their driver has gone
too) add dummy functions.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: kbuild test robot <fengguang.wu@intel.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: cpufreq@vger.kernel.org
Cc: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>