Add support for i6s radio

[ghouet]

Before suggesting a new feature here, please search the open issue if there is already a request for the feature

Is your feature request related to a problem? Please describe.
The i6s radio has the same stm32 chip. I tried flashing this i6x opentx firmware but it says bad eeprom when starting the radio.
The i6s has an easy way for DFU mode: https://fishpepper.de/2016/09/15/openground-part-2-fs-i6s-dfu-bootloader-access/

Describe the solution you'd like
Make this project work with i6s radio.

Describe alternatives you've considered

Additional context

[ajjjjjjjj]

We don't have that device. You have to wait for someone with i6S to contribute to the project or take take action by yourself! ;)

[DetourFPV]

If someone is working on this in Colorado I can provide a loaner i6S for some testing, but I'd like to have it back...preferably running EdgeTX and ExpressLRS.

[DaniilVeselov]

https://github.com/fishpepper/OpenGround

Maybe it will be helpful

[ajjjjjjjj]

Closing in favor of #386