dt-bindings: timer: mtk-timer: add support for MT8516

Add binding documentation of mtk-timer for MT8516 SoC.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>