mlxsw: Introduce Mellanox SwitchX-2 ASIC support

Benefit from the previously introduced Mellanox Switch infrastructure and
add driver for SwitchX-2 ASIC. Note that this driver is very simple now.
It implements bare minimum for getting device to work on slow-path.
Fast-path offload functionality is going to be added soon.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Elad Raz <eladr@mellanox.com>
Reviewed-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>