dt-bindings: net: macb: Add mdio bus child node

This adds an optional mdio bus child node. If present, the mac will
look for PHYs there instead of directly under the top-level node. This
eliminates any ambiguity about whether child nodes are PHYs, and allows
the MDIO bus to contain non-PHY devices.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>