cpufreq: mediatek: Add opp notification support

From this opp notifier, cpufreq should listen to opp notification and do
proper actions when receiving events of disable and voltage adjustment.

One of the user for this opp notifier is MediaTek SVS.
The MediaTek Smart Voltage Scaling (SVS) is a hardware which calculates
suitable SVS bank voltages to OPP voltage table.

Signed-off-by: Andrew-sh.Cheng <andrew-sh.cheng@mediatek.com>
Signed-off-by: Jia-Wei Chang <jia-wei.chang@mediatek.com>
Signed-off-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
[ Viresh: Renamed opp_freq as current_freq and moved its initialization ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>