mlxsw: spectrum: Add support for port identification

Allow a user to flash the port's LED in order to identify it. This is
achieved by setting the Management LED Control Register (MLCR).

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>