dt-bindings: net: dsa: add new MT7531 binding to support MT7531

Add devicetree binding to support the compatible mt7531 switch as used
in the MediaTek MT7531 switch.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Landen Chao <landen.chao@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>