thermal/of: Move thermal_trip structure to thermal.h

The structure thermal_trip is now generic and will be usable by the
different sensor drivers in place of their own structure.

Move its definition to thermal.h to make it accessible.

Cc: Alexandre Bailon <abailon@baylibre.com>
Cc: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linexp.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://lore.kernel.org/r/20220722200007.1839356-5-daniel.lezcano@linexp.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>