mlxsw: core: Allow thermal zone binding to an external cooling device

Allow thermal zone binding to an external cooling device from the
cooling devices white list.

It provides support for Mellanox next generation systems on which
cooling device logic is not controlled through the switch registers.

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