[chirp_devel] Adding values to STEPS list

Hello to the team; thank you very much for providing this tool!

I'm new to chirp, not to SW devel.

In programming my new Baofeng F8HP, I was surprised that 15 kHz wasn't a valid step value, since that is the standard US channel spacing on 2m.

In looking through the drivers, the STEP values seem to be very radio specific for reasons I don't yet understand.  Before I put in a bug and patch to add 15kHz to the uv3r driver, I need to ask whether there a specific reason it isn't already in the list which makes it incorrect to add it?

Thanks, Al WA1TYB