dt-bindings: mmc: eliminate yamllint warnings

Eliminate the following yamllint warnings:
./Documentation/devicetree/bindings/mmc/mtk-sd.yaml
:20:9: [warning] wrong indentation: expected 10 but found 8 (indentation)
:30:9: [warning] wrong indentation: expected 10 but found 8 (indentation)
:33:9: [warning] wrong indentation: expected 10 but found 8 (indentation)

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201207032626.1445-2-thunder.leizhen@huawei.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/mtk-sd.yaml

@@ -17,21 +17,21 @@ properties:
   compatible:
     oneOf:
       - enum:
-        - mediatek,mt2701-mmc
-        - mediatek,mt2712-mmc
-        - mediatek,mt6779-mmc
-        - mediatek,mt7620-mmc
-        - mediatek,mt7622-mmc
-        - mediatek,mt8135-mmc
-        - mediatek,mt8173-mmc
-        - mediatek,mt8183-mmc
-        - mediatek,mt8516-mmc
+          - mediatek,mt2701-mmc
+          - mediatek,mt2712-mmc
+          - mediatek,mt6779-mmc
+          - mediatek,mt7620-mmc
+          - mediatek,mt7622-mmc
+          - mediatek,mt8135-mmc
+          - mediatek,mt8173-mmc
+          - mediatek,mt8183-mmc
+          - mediatek,mt8516-mmc
       - items:
-        - const: mediatek,mt7623-mmc
-        - const: mediatek,mt2701-mmc
+          - const: mediatek,mt7623-mmc
+          - const: mediatek,mt2701-mmc
       - items:
-        - const: mediatek,mt8192-mmc
-        - const: mediatek,mt8183-mmc
+          - const: mediatek,mt8192-mmc
+          - const: mediatek,mt8183-mmc
 
   clocks:
     description: