As said, I've looked on Google, looked at the srv controls, videos, nothing will lead me to the control for this turret, FD sure did a job making it hard to find the turret exit easy to find so you can bind it.
You're not scrolling down far enough. It's at the very bottom under all the roll and pitch controls in the same area as the Handbrake and Headlights. After you open the Driving extended menu you need to keep scrolling until you see the Driving Targeting extended menu and then go up one position.
When 2.0 first hit and I got into the SRV, there was a bug that caused all of your driving controls to be unmapped. I feel your pain OP, glad you found it in the end!