ath9k: add bits definition of BTCoex MODE2/3 for SOC chips

Add bits definition for AR_BT_COEX_MODE2 and AR_BT_COEX_MODE3, which
needed by SOC chips (AR9340, AR9531, AR9550, AR9561).

Signed-off-by: Miaoqing Pan <miaoqing@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>