dt-bindings: mediatek: update bindings for MT8183 systimer

This commit adds mt8183 compatible node in mtk-timer binding document.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dehui Sun <dehui.sun@mediatek.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>