Hello,
Mbed TLS does not currently support SRP and it is not on our roadmap (https://developer.trustedfirmware.org/w/mbed-tls/roadmap/). Arm does not intend to work on it, but support can be added if someone else contributes it.
If you are interested in contributing SRP support, please discuss it on this list first to settle some potential issues: conflicts with other work (in particular TLS 1.3 preparation, which involves some refactoring of existing TLS code), review bandwidth schedule, test plan.
mbed-tls@lists.trustedfirmware.org