mbedtls_platform_setup and psa_crypto_init

### Description

Work out how `mbedtls_platform_setup()` and `psa_crypto_init()` should interact.

See discussion at https://github.com/ARMmbed/mbed-os-example-tls/pull/232#discussion_r249813184

Some scenarios to consider:

* As an application writer, I'm using Mbed TLS for TLS and I don't care about Mbed Crypto.
* As an application writer, I'm using the PSA crypto API and I don't care about how it's implemented.
* As an application writer, I'm combining one module that uses Mbed TLS and one that uses PSA crypto.
* … and my application is multithreaded.
* As an operating system integrator, I provide my own version of `mbedtls_platform_setup`.
* As a device integrator, I'm using Mbed OS and I don't care what various bits may be doing under the hood.

### Issue request type

[ ] Question
[X] Enhancement
[ ] Bug

Start with the interaction between mbedtls_platform_setup() and psa_crypto_init(), then read the linked discussion in mbed-os-example-tls pull request 232. A complete result would decide and document behavior for the listed application, multithreading, operating-system, and device-integration scenarios. The repository states that development has moved to Mbed TLS and that no further updates will be made here.

Written by the indexing model from the issue text.