Looks like the Flint 3 is working its way toward OpenWRT support along with other Qualcomm IPQ5332 based routers. Below is a post on the OpenWRT forum showing progress made and hurdles yet to be overcome. It would be great to see GL.iNet lend support to the effort!
We have confirmed with Realtek that, since the RTL8372N driver has not been open-sourced, we are unfortunately unable to provide the corresponding documentation or source code. However, we are actively encouraging them to move this forward.
Regarding the GL Board DTS, we believe it may be of limited use if the switch driver itself cannot be provided. That said, if you still need it, please let us know.
Please also feel free to let us know if there is any other assistance we can provide.