Support for SCS 15 kHz

## Description
I have received a request from the community to support spacing of 15kHz. According to ETSI 103720 this is one of the values that is possible: 

<img width="914" alt="Bildschirmfoto 2024-07-25 um 11 17 40" src="https://github.com/user-attachments/assets/96040f77-b510-4649-9934-7849150d5373">
