arm64: dts: mediatek: add mt8173 elm and hana board

Elm is Acer Chromebook R13. Hana is Lenovo Chromebook. Both uses mt8173
SoC.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20200210063523.133333-5-hsinyi@chromium.org


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