mlxsw: core_hwmon: Add interfaces for line card initialization and de-initialization

Add callback functions for line card 'hwmon' initialization and
de-initialization. Each line card is associated with the relevant
'hwmon' device, which may contain thermal attributes for the cages
and gearboxes found on this line card.

The line card 'hwmon' initialization / de-initialization APIs are to be
called when line card is set to active / inactive state by
got_active() / got_inactive() callbacks from line card state machine.

For example cage temperature for module #9 located at line card #7 will
be exposed by utility 'sensors' like:
linecard#07
front panel 009:	+32.0C  (crit = +70.0C, emerg = +80.0C)
And temperature for gearbox #3 located at line card #5 will be exposed
like:
linecard#05
gearbox 003:		+41.0C  (highest = +41.0C)

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>