Point OSS-Fuzz to TF-PSA-Crypto

[gilles-peskine-arm]

Once TF-PSA-Crypto hosts crypto, we should migrate the OSS-Fuzz crypto fuzzing from Mbed TLS to TF-PSA-Crypto. Mbed TLS is fuzzed via four projects in OSS-Fuzz:

• mbedtls — pk, x509 and ssl fuzzing. Stays in mbedtls, unless we want to migrate the pk fuzzers?
• bignum-fuzzer — if it's still relevant, as its functionality is partly subsumed by cryptofuzz?
• ecc-diff-fuzzer — if it's still relevant, as its functionality is partly subsumed by cryptofuzz?
• cryptofuzz

This can be done before or after the 1.0 release: it's ok if the fuzzers keep fuzzing crypto APIs as they are re-exported by Mbed TLS. But we may want to combine this with Mbed-TLS/mbedtls#9071.