IB/mlx5: Add basic regiser/unregister representors code

Create the basic infrastructure of registering and unregistering
IB representors. The load/unload callbacks are left empty and
proper implementation will be introduced in following patches.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>