dt: emac: document device-tree based phy discovery and setup

dt: emac: document device-tree based phy discovery and setup

This patch adds documentation for a new "phy-handle" property,
"fixed-link" and "mdio" sub-node. These allows the enumeration
of PHYs which are supported by the phy library under drivers/net/phy.

The EMAC ethernet controller in IBM and AMCC 4xx chips is
currently stuck with a few privately defined phy
implementations. It has no support for PHYs which
are supported by the generic phylib.

Acked-by: Rob Herring <[email protected]>
Reviewed-by: Florian Fainelli <[email protected]>
Signed-off-by: Christian Lamparter <[email protected]>
Signed-off-by: David S. Miller <[email protected]>