Remove mbedtls_pk_type_t from the API

[gilles-peskine-arm]

Move the parts of pk.h that are becoming private in TF-PSA-Crypto out of pk.h and ensure that they are no longer documented as public APIs. This notably includes mbedtls_pk_type_t. Adjust PK and consuming APIs in Mbed TLS accordingly.

See #203 for the architectural specification, including the list of all the API elements made private, and how to make mbedtls_pk_type_t private.

Removing the use of mbedtls_pk_type_t in library code, test code and sample programs is out of scope and will likely not be done until after TF-PSA-Crypto 1.0 is released.

Where there are changes to the specification of existing functions, unit tests for the new behavior is out of scope of this issue, and will be done in a follow-up #207.