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>

Documentation/devicetree/bindings/net/macb.txt

@@ -30,6 +30,10 @@ Required properties:
 	Optional elements: 'tsu_clk'
 - clocks: Phandles to input clocks.
 
+Optional properties:
+- mdio: node containing PHY children. If this node is not present, then PHYs
+        will be direct children.
+
 The MAC address will be determined using the optional properties
 defined in ethernet.txt.