thermal: mtk_thermal: make device_reset optional

MT8516 does not support thermal reset. Use device_reset_optional
instead of device_reset.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201021164231.3029956-3-fparent@baylibre.com