The files within the p256-m/ subdirectory originate from the p256-m GitHub repository, which is distributed under the Apache 2.0 license. They are authored by Manuel Pégourié-Gonnard. p256-m is a minimalistic implementation of ECDH and ECDSA on NIST P-256, especially suited to constrained 32-bit environments. Mbed TLS documentation for integrating drivers uses p256-m as an example of a software accelerator, and describes how it can be integrated alongside Mbed TLS.
The files p256-m.c and .h, along with the license, have been taken from the p256-m repository.
It should be noted that p256-m deliberately does not supply its own cryptographically secure RNG function. As a result, an RNG function using mbedtls_ctr_dbrg has been implemented and added to p256m.c.