dt-bindings: mmc: Add support for Mediatek MT7986

This commit adds dt-binding documentation of mmc for Mediatek MT7986 SoC
Platform.
Add SoC specific section for defining clock configuration.

Signed-off-by: Sam Shih <sam.shih@mediatek.com>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221025132953.81286-3-linux@fw-web.de


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