net/mlx5e: Add netdev support for VXLAN tunneling

If a VXLAN udp dport is added to device it will:

   - Configure the hardware to offload the port (up to the max
     supported).
   - Advertise NETIF_F_GSO_UDP_TUNNEL and supported hw_enc_features.

Signed-off-by: Matthew Finlay <matt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>