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

Documentation/devicetree/bindings/timer/mediatek,mtk-timer.txt

@@ -25,6 +25,7 @@ Required properties:
 	For those SoCs that use SYST
 	* "mediatek,mt8183-timer" for MT8183 compatible timers (SYST)
 	* "mediatek,mt8186-timer" for MT8186 compatible timers (SYST)
+	* "mediatek,mt8188-timer" for MT8188 compatible timers (SYST)
 	* "mediatek,mt8192-timer" for MT8192 compatible timers (SYST)
 	* "mediatek,mt8195-timer" for MT8195 compatible timers (SYST)
 	* "mediatek,mt7629-timer" for MT7629 compatible timers (SYST)