wifi: rtw89: mac: add function to get TSF

Add mac function rtw89_mac_port_get_tsf() to get TSF by port.
It will be used when MCC (multi-channel concurrency) calculates
timing things.

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/20230119062453.58341-4-pkshih@realtek.com