thermal: stm32: Handle multiple trip points

Let the thermal framework handle the trip points instead
of custom code inside the driver. This is backward compatible,
simplifies the driver and offers the possibility to the user
to set any trip point he needs.

stm_thermal_set_trips callback that is registered to
set_trips ops to handle the low and high thresholds and replaces
stm_thermal_set_threshold and stm_thermal_update_threshold functions.
modify irq enable to handle the thresholds.

Signed-off-by: Pascal Paillet <p.paillet@st.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200110101605.24984-5-p.paillet@st.com