thermal: core: Relocate the struct thermal_governor definition

Notice that struct thermal_governor is only used by the thermal core
and so move its definition to thermal_core.h.

No functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>