The PSA crypto code needs mbedtls_pk_write_key_der() and
mbedtls_pk_write_pubkey() when using RSA without drivers. We were already
forcing MBEDTLS_PK_WRITE_C when MBEDTLS_USE_PSA_CRYPTO is enabled. Do so
also when MBEDTLS_PSA_CRYPTO_C is enabled as well as MBEDTLS_RSA_C, even
without MBEDTLS_USE_PSA_CRYPTO. Fixes#6408.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
