net/mlx5: Add MACsec offload Tx command support

This patch adds support for Connect-X MACsec offload Tx SA commands:
add, update and delete.

In Connect-X MACsec, a Security Association (SA) is added or deleted
via allocating a HW context of an encryption/decryption key and
a HW context of a matching SA (MACsec object).

When new SA is added:
- Use a separate crypto key HW context.
- Create a separate MACsec context in HW to include the SA properties.

Introduce a new compilation flag MLX5_EN_MACSEC for it.

Follow-up patches will implement the Tx steering.

Signed-off-by: Lior Nahmanson <liorna@nvidia.com>
Reviewed-by: Raed Salem <raeds@nvidia.com>
Signed-off-by: Raed Salem <raeds@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>