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>

Documentation/devicetree/bindings/arm/mediatek.txt

@@ -15,8 +15,7 @@ compatible: Must contain one of
    "mediatek,mt6795"
    "mediatek,mt6797"
    "mediatek,mt7622"
-   "mediatek,mt7623" which is referred to MT7623N SoC
-   "mediatek,mt7623a"
+   "mediatek,mt7623"
    "mediatek,mt8127"
    "mediatek,mt8135"
    "mediatek,mt8173"