net/mlx5: E-Switch, Add control for encapsulation

Implement the devlink e-switch encapsulation control set and get
callbacks. Apply the value set by the user on the switchdev offloads
mode when creating the fast FDB table where offloaded rules will be set.

Signed-off-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>