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

Documentation/devicetree/bindings/arm/mediatek.yaml

@@ -84,6 +84,28 @@ properties:
           - enum:
               - mediatek,mt8135-evbp1
           - const: mediatek,mt8135
+      - description: Google Elm (Acer Chromebook R13)
+        items:
+          - const: google,elm-rev8
+          - const: google,elm-rev7
+          - const: google,elm-rev6
+          - const: google,elm-rev5
+          - const: google,elm-rev4
+          - const: google,elm-rev3
+          - const: google,elm
+          - const: mediatek,mt8173
+      - description: Google Hana (Lenovo Chromebook N23 Yoga, C330, 300e,...)
+        items:
+          - const: google,hana-rev6
+          - const: google,hana-rev5
+          - const: google,hana-rev4
+          - const: google,hana-rev3
+          - const: google,hana
+          - const: mediatek,mt8173
+      - description: Google Hana rev7 (Poin2 Chromebook 11C)
+        items:
+          - const: google,hana-rev7
+          - const: mediatek,mt8173
       - items:
           - enum:
               - mediatek,mt8173-evb