Clarify the status and possibly rename of intended-private PSA_xxx and psa_xxx identifiers

[gilles-peskine-arm]

PSA crypto headers define a number of PSA_xxx macro names that are not part of the PSA specification. Originally, the identifiers that were part of the specification were exactly the identifiers that had Doxygen documentation, although it's possible that there has been some divergence.

We should also make it clear which names are part of the Mbed TLS API and which ones are internal names that may change in future versions.

Goal of this task:

• Make it clear in the Mbed TLS documentation (both when reading headers and when reading Doxygen documentation) which identifiers are from a PSA standard, which are specific to the Mbed TLS API, and which are specific to the current version of Mbed TLS.
• Possibly rename identifers that are not PSA standard or PSA standard track to start with MBEDTLS_xxx.
• Possibly rename functions and types that are not PSA standard (track) to mbedtls_xxx.

[gilles-peskine-arm]

What we should do in TF-PSA-Crypto 1.0:

• Rename all the mbedtls_xxx functions and types defined in psa/*.h to tfc_xxx.
• Also, consistently, rename the psa_xxx that are not on track for PSA standardization.
• Also a few types need to be renamed.

[gilles-peskine-arm]

Preferably we should also ensure that it's clear to users which macros are part of the public API and which aren't. In particular, undocumented macros should not be included in Doxygen.