net: sockets: tls: Add config for DTLS max fragment length

Add CONFIG_NET_SOCKETS_DTLS_MAX_FRAGMENT_LENGTH for limiting
the Maximum Fragment Length (MFL) for DTLS with Mbed TLS.

This is needed when MBEDTLS_SSL_OUT_CONTENT_LEN and
MBEDTLS_SSL_IN_CONTENT_LEN are set to larger values than the MTU
of the network and IP fragmentation is not supported.

Signed-off-by: Markus Lassila <markus.lassila@nordicsemi.no>