dt-bindings: timer: Add compatible for MediaTek MT8188

Add dt-binding documentation of timer for MediaTek MT8188 SoC
platform.

Signed-off-by: Johnson Wang <johnson.wang@mediatek.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220818124602.20394-1-johnson.wang@mediatek.com


Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>