thermal: Allow selecting the bang-bang governor as default

For many setups the bang-bang governor is exactly what we want. Many
ARM SoC-based devices use fans to cool down the entire SoC and that
works well only with the bang-bang governor because it uses the
hysteresis in order to let the fan run for a while to cool the SoC
down below the trip point before switching it off again.

The step-wise governor will behave strangely in these situations. It
doesn't use the hysteresis, so it can lead to situations where the fan
is turned on for only a very brief period and then is switched back off,
only to get switched back on again very quickly because the SoC hasn't
cooled down very much.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20230609124408.3788680-1-thierry.reding@gmail.com
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>