Hi Mathieu,
To integrate a driver, the idea is to instruct the build system what drivers are available, and a script will generate .c and .h files with the required declarations and dispatch code. https://github.com/Mbed-TLS/mbedtls/blob/development/docs/proposed/psa-drive... is a draft of how this will look like. Unfortunately, we have only just started to implement this (the current work in progress is https://github.com/Mbed-TLS/mbedtls/pull/5396). So in the meantime, you need to edit library/psa_crypto_driver_wrappers.c and include/psa/crypto_driver_contexts*.h manually.
We currently have no plans for managing the upstreaming of drivers. One reason is that we're a bit uncomfortable hosting code that we can't test, and setting up testing for all the hardware is a big job in itself. Another reason is that driver code tends to depend on the operating system or runtime environment — it needs to map memory, react to interrupts, etc.
For those reasons, my personal opinion is that rather than Mbed TLS managing driver upstreaming, this will be done by platforms such as Mbed OS, Zephyr, etc. Mbed OS, at least, has already included many Mbed TLS ALT implementations for years, and will gradually include PSA crypto drivers.
Best regards,