dt-bindings: net: ftgmac100: Document AST2600 compatible

The AST2600 contains an FTGMAC100-compatible MAC, although the MDIO
controller previously embedded in the MAC has been moved out to a
dedicated MDIO block.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Joel Stanley <joel@jms.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/ftgmac100.txt

@@ -9,6 +9,7 @@ Required properties:
 
      - "aspeed,ast2400-mac"
      - "aspeed,ast2500-mac"
+     - "aspeed,ast2600-mac"
 
 - reg: Address and length of the register set for the device
 - interrupts: Should contain ethernet controller interrupt