thermal/drivers/of-thermal: Move the of_thermal_free_zone() to the init section

The function of_thermal_free_zone() is only used the initialization
function which all belonging to the init section.

Move it also to the __init section.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20191219222154.16100-2-daniel.lezcano@linaro.org