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>