dt-bindings: timer: mediatek: update bindings for MT7629 SoC

Update the binding for MT7629 SoC, which uses fallback compatible to
MT6765 SYST, so add more descriptions to distinguish it from the other
SoCs that use GPT.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>