arm64: dts: mt8173: add timer node

Add device node to enable GPT timer.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Eddie Huang <eddie.huang@mediatek.com>
Signed-off-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>