Add support for tasmota-flashed Sonoff ZBBridge (EFR32MG21 router)

[alichaudry]

As discussed here.

Not sure if the UT-01 model I added is problematic as I reused the UT-02 model here.

Also open to any other changes that may need to go in.

[alichaudry]

Hmm, looks like the liner failed on ikea.ts, a file I didn't touch. Not sure if a recent PR broke that or if I need to do something in my PR to address this.

[Koenkk]

Thanks!

Could you also submit a picture for the docs? This can be done by:

1. Create a fork by clicking here
2. Go to the public/images/devices directory, Add file -> Upload files
3. Upload the files and press Commit changes
4. Press Contribute -> Open pull request -> update title/description -> Create pull request

Make sure that:

• The filename is EFR32MG21.Router.png
• The size is 512x512
• The background is transparent (use e.g. Adobe remove background)