dt-bindings: mmc: Convert mtk-sd to json-schema

Convert the mtk-sd binding to DT schema format using json-schema.

Signed-off-by: Wenbin Mei <wenbin.mei@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201014030846.12428-2-wenbin.mei@mediatek.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>