dt-bindings: arm64: dts: mediatek: Add mt8173 elm and hana

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

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200210063523.133333-2-hsinyi@chromium.org


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