mbedtls: default enable USE_PSA_CRYPTO when CRYPTO_C

Now that support for all PSA_WANT symbols is in place, we can
enable MBEDTLS_USE_PSA_CRYPTO when MBEDTLS_PSA_CRYPTO_C is
enabled as well.

Note: this commit also moves USE_PSA_CRYPTO out of CRYPTO_C
dependency in config-tls-generic.h because TLS/DTLS/X509 modules
of MbedTLS can rely on *any* implementation of PSA crypto APIs
not only the MbedTLS one. TFM is for example an alternative
to this.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>