dt-bindings: mmc: Add compatible for Mediatek MT8186

This commit adds dt-binding documentation of mmc for Mediatek MT8186 SoC
Platform.

Signed-off-by: Allen-KH Cheng <Allen-KH.Cheng@mediatek.com>
Link: https://lore.kernel.org/r/20220128062050.23978-5-allen-kh.cheng@mediatek.com


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