mlxsw: core: Extend allowed list of external cooling devices for thermal zone binding

Extend the list of allowed external cooling devices for thermal zone
binding to include devices of type "emc2305".

The motivation is to provide support for the system SN2201, which is
equipped with the Spectrum-1 ASIC.
The system's airflow control is managed by the EMC2305 RPM-based PWM
Fan Speed Controller as the cooling device.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>