PM / devfreq: Export devfreq_get_freq_range symbol within devfreq

In order to get frequency range within devfreq governors,
export devfreq_get_freq_range symbol within devfreq.

Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Tested-by: Johnson Wang <johnson.wang@mediatek.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>