cpufreq: mediatek: change to regulator_get_optional

For new mediatek chip mt8183,
cci and little cluster share the same buck,
so need to modify the attribute of regulator from exclusive to optional

Signed-off-by: Andrew-sh.Cheng <andrew-sh.cheng@mediatek.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>