dt-bindings: arm: mediatek: cleanup MT7623N reference boards

Cleanup binding document to get rid of unsupported reference boards
for MT7623N.

Cc: John Crispin <john@phrozen.org>
Cc: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Acked-by: John Crispin <john@phrozen.org>
Acked-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>