mlxsw: core_hwmon: Extend hwmon interface with critical and emergency alarms

Add new attributes to hwmon object for exposing critical and emergency
alarms.

In case that current temperature is higher than emergency threshold,
EMERGENCY alarm will be reported in sensors utility:

$ sensors
...
front panel 025:  +55.0°C  (crit = +35.0°C, emerg = +40.0°C) ALARM(EMERGENCY)

In case that current temperature is higher than critical threshold,
CRIT alarm will be reported in sensors utility:

$ sensors
...
front panel 025:  +54.0°C  (crit = +35.0°C, emerg = +80.0°C) ALARM(CRIT)

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