dt-bindings: net: dsa: add new MT7530 binding to support MT7621

Add devicetree binding to support the compatible mt7530 switch as used
in the MediaTek MT7621 SoC.
Signed-off-by: Greg Ungerer <gerg@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Sean Wang <sean.wang@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -3,12 +3,16 @@ Mediatek MT7530 Ethernet switch
 
 Required properties:
 
-- compatible: Must be compatible = "mediatek,mt7530";
+- compatible: may be compatible = "mediatek,mt7530"
+	or compatible = "mediatek,mt7621"
 - #address-cells: Must be 1.
 - #size-cells: Must be 0.
 - mediatek,mcm: Boolean; if defined, indicates that either MT7530 is the part
 	on multi-chip module belong to MT7623A has or the remotely standalone
 	chip as the function MT7623N reference board provided for.
+
+If compatible mediatek,mt7530 is set then the following properties are required
+
 - core-supply: Phandle to the regulator node necessary for the core power.
 - io-supply: Phandle to the regulator node necessary for the I/O power.
 	See Documentation/devicetree/bindings/regulator/mt6323-regulator.txt