mlxsw: reg: Add Management Fan Speed Limit register

The MFSL register is used to configure the fan speed event / interrupt
notification mechanism. Fan speed threshold are defined for both
under-speed and over-speed.

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