dt-bindings: mmc: eliminate yamllint warnings

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>