thermal/drivers/of-thermal: Make of_thermal_destroy_zones static

The function of_thermal_destroy_zones() is only used internally by the
of_parse_thermal_zones() for rollbacking in case of error.

Make it static and tag it as an __init function.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20191219222154.16100-1-daniel.lezcano@linaro.org