thermal: core: Make thermal_zone_set_trips private

The function thermal_zone_set_trips() is used by the thermal core code
in order to update the next trip points, there are no other users.

Move the function definition in the thermal_core.h, remove the
EXPORT_SYMBOL_GPL and document the function.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Link: https://lore.kernel.org/r/20200331165449.30355-1-daniel.lezcano@linaro.org