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 <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>