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>

Documentation/devicetree/bindings/net/dsa/mt7530.txt

@@ -5,6 +5,7 @@ Required properties:
 
 - compatible: may be compatible = "mediatek,mt7530"
 	or compatible = "mediatek,mt7621"
+	or compatible = "mediatek,mt7531"
 - #address-cells: Must be 1.
 - #size-cells: Must be 0.
 - mediatek,mcm: Boolean; if defined, indicates that either MT7530 is the part
@@ -32,10 +33,14 @@ Required properties for the child nodes within ports container:
 
 - reg: Port address described must be 6 for CPU port and from 0 to 5 for
 	user ports.
-- phy-mode: String, must be either "trgmii" or "rgmii" for port labeled
-	 "cpu".
-
-Port 5 of the switch is muxed between:
+- phy-mode: String, the following values are acceptable for port labeled
+	"cpu":
+	If compatible mediatek,mt7530 or mediatek,mt7621 is set,
+	must be either "trgmii" or "rgmii"
+	If compatible mediatek,mt7531 is set,
+	must be either "sgmii", "1000base-x" or "2500base-x"
+
+Port 5 of mt7530 and mt7621 switch is muxed between:
 1. GMAC5: GMAC5 can interface with another external MAC or PHY.
 2. PHY of port 0 or port 4: PHY interfaces with an external MAC like 2nd GMAC
    of the SOC. Used in many setups where port 0/4 becomes the WAN port.