dt-bindings: timer: Add compatible for Mediatek MT8186

This commit adds dt-binding documentation of timer for Mediatek MT8186 SoC
Platform.

Signed-off-by: Allen-KH Cheng <Allen-KH.Cheng@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220311130732.22706-2-allen-kh.cheng@mediatek.com
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>