net/mlx5: Move all devlink related functions calls to devlink.c

Centralize all devlink related callbacks in one file.
In the downstream patch, some more functionality will be added, this
patch is preparing the driver infrastructure for it.

Currently, move devlink un/register functions calls into this file.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Reviewed-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>