wifi: rtw89: add function prototype for coex request duration

The request duration comes from coex mechanism, indicating the
length of time that should be reserved for BT in each time division.
It is required to handle update notification when channel concurrency
processes. Since it will involve in both coex and wifi code flow, this
commit ahead adds the prototype for required function interfaces to
split the implementation of coex and wifi in the following.

The follow-up are expected be add afterwards.
1. coex mechanism call rtw89_core_ntfy_btc_event() once bt req len changes
2. channel concurrency flow updates related stuffs when notified

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230816082133.57474-2-pkshih@realtek.com