thermal/core: Add thermal_zone_device structure 'type' accessor

The thermal zone device structure is exposed via the exported
thermal.h header. This structure should stay private the thermal core
code. In order to encapsulate the structure, let's add an accessor to
get the 'type' of the thermal zone.

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