dt-bindings: thermal: mediatek: make resets property optional

MT8516 Thermal IP does not support reset. Make the resets property
optional in order to be able to support MT8516 SoC.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201021164231.3029956-1-fparent@baylibre.com