arm64: dts: mt8173: add uart aliases

Add serial as uart aliases in mt8173.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20200210063523.133333-3-hsinyi@chromium.org


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>