dt-bindings: thermal: mediatek: add documentation for MT8516 SoC

Add binding documentation for the MediaTek MT8516 SoC.
The SoC thermal IP is similar to MT2701.

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/20201021164231.3029956-2-fparent@baylibre.com