thermal/drivers/exynos: of_thermal_get_ntrips()

The thermal core framework allows to get the number of thermal trips,
use it instead of visiting the thermal core structure internals.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221003092602.1323944-9-daniel.lezcano@linaro.org