dt-bindings: mmc: Add compatible for MediaTek MT8188

This commit adds dt-binding documentation of mmc for MediaTek MT8188 SoC
platform.

Signed-off-by: Johnson Wang <johnson.wang@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220708114747.13878-1-johnson.wang@mediatek.com


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