dt-bindings: net: Add aspeed, ast2600-mdio binding

The AST2600 splits out the MDIO bus controller from the MAC into its own
IP block and rearranges the register layout. Add a new binding to
describe the new hardware.

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