mlxsw: core: Set different thermal polling time based on bus frequency capability

Add low frequency bus capability in order to allow core functionality
separation based on bus type. Driver could run over PCIe, which is
considered as high frequency bus or I2C, which is considered as low
frequency bus. In the last case time setting, for example, for thermal
polling interval, should be increased.

Use different thermal monitoring based on bus type. For I2C bus time is
set to 20 seconds, while for PCIe 1 second polling interval is used.

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>