dt-bindings: arm: mediatek: remove unused "mediatek, mt7623a"

As we fallback to use "mediatek,mt7623" for MT7623a, remove unused
root node property "mediatek,mt7623a" in the document.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>