mlxsw: spectrum: Introduce port splitting

Allow a user to split or unsplit a port using the newly introduced
devlink ops.

Once split, the original netdev is destroyed and 2 or 4 others are
created, according to user configuration. The new ports are like any
other port, with the sole difference of supporting a lower maximum
speed. When unsplit, the reverse process takes place.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>