arm: dts: mt7623: update mmc related nodes with the appropriate fallback

The current mmc related nodes should be falling back to MT2701
as the dt-binding defines and which has more appropriate setup
for MT7623.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>