mtk_ccifreq_opp_notifier is reenter function when policy0 / policy6
change freq, so mutex_lock should protect all OPP event.

Link: https://lore.kernel.org/lkml/20230818021741.6173-1-chun-jen.tseng@mediatek.com/T/Signed-off-by: Mark Tseng <chun-jen.tseng@mediatek.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>