dt-bindings: mmc: mediatek: Add document for mt6779

Add compatible node for mt6779 mmc.

Signed-off-by: Chun-Hung Wu <chun-hung.wu@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1595205759-5825-5-git-send-email-chun-hung.wu@mediatek.com


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