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

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

The line card thermal 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 thermal zone for module #9 located at line card #7 will
have type:
mlxsw-lc7-module9.
And thermal zone for gearbox #2 located at line card #5 will have type:
mlxsw-lc5-gearbox2.

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>