Where did you get the driver from?
If you're using the new fbtft driver with device tree enabled then you would probably use something like
where xxx is the name of the overlay.
If you load the driver in /etc/modules then you would add the 'rotate' to the driver / module name (or use a separate conf file).
And if you're looking for the source code for fbtft: see https://git.kernel.org/cgit/linux/kerne ... ng-testing