thermal/of: Unexport unused OF functions

The functions thermal_of_zone_register() and
thermal_of_zone_unregister() are no longer needed from the drivers as
the devm_ variant is always used.

Make them static in the C file and remove their declaration from thermal.h

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230404075138.2914680-2-daniel.lezcano@linaro.org