rtw89: pci: add variant RPWM/CPWM to enter low power mode

RPWM/CPWM are registers that can set and check low power mode. Since chips
use different address, add a field to access them in common flow.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220421120903.73715-3-pkshih@realtek.com