dt-bindings: clock: mediatek: Add new MT8188 clock

Add the new binding documentation for system clock
and functional clock on MediaTek MT8188.

Signed-off-by: Garmin.Chang <Garmin.Chang@mediatek.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230331123621.16167-2-Garmin.Chang@mediatek.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>