thermal: core: Encapsulate tz->device field

thermal: core: Encapsulate tz->device field

There are still some drivers needing to play with the thermal zone
device internals. That is not the best but until we can figure out if
the information is really needed, let's encapsulate the field used in
the thermal zone device structure, so we can move forward relocating
the thermal zone device structure definition in the thermal framework
private headers.

Some drivers are accessing tz->device, that implies they need to have
the knowledge of the thermal_zone_device structure but we want to
self-encapsulate this structure and reduce the scope of the structure
to the thermal core only.

By adding this wrapper, these drivers won't need the thermal zone
device structure definition and are no longer an obstacle to its
relocation to the private thermal core headers.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>