Hide 192-bit curves

See https://github.com/Mbed-TLS/mbedtls/issues/8136 - for 192-bit curves, as a first step we only want to hide them (fully removing them would require re-generating lots of test data in order not to lose coverage).

Depends on https://github.com/Mbed-TLS/TF-PSA-Crypto/issues/233