mlxsw: reg: Add high and low temperature thresholds

The ASIC has the ability to generate events whenever a sensor indicates
the temperature goes above or below its high or low thresholds,
respectively.

In new firmware versions the firmware enforces a minimum of 5
degrees Celsius difference between both thresholds. Make the driver
conform to this requirement.

Note that this is required even when the events are disabled, as in
certain systems interrupts are generated via GPIO based on these
thresholds.

Fixes: 85926f87 ("mlxsw: reg: Add definition of temperature management registers")
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>