mlxsw: core: Add an infrastructure to track transceiver overheat counter

Initialize an array that stores per-module overheat state and a counter
indicating how many times the module was in overheat state.

Export a function to query the counter according to module number.
Will be used later on by the switch driver (i.e., mlxsw_spectrum) to expose
module's overheat counter as part of ethtool statistics.

Initialize mlxsw_env after driver initialization to be able to query
number of modules from MGPIR register.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>