wifi: rtw89: add XTAL SI for WiFi 7 chips

The XTAL SI is a serial interface to indirectly access registers of
analog hardware circuit. Since WiFi 7 chips use different registers, add
a ops to access them via common functions. This patch doesn't change logic
for existing chips.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20231211083341.118047-2-pkshih@realtek.com