wcn36xx: Add a chip identifier for WCN3680

The WCN3680 has some specific behaviours that we want to capture to
distinguish it from the WCN3620 and WCN3660 respectively.

Add an identifier for this purpose.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200829033846.2167619-2-bryan.odonoghue@linaro.org