ethoc: Add device tree configuration

This patch adds the ability to describe ethernet devices via a flattened
device tree.  As device tree remains an optional feature, these bits all
need to be guarded by CONFIG_OF ifdefs.

MAC address is settable via the device tree parameter "local-mac-address";
however, the selection of the phy id is limited to probing, for now.

Signed-off-by: Jonas Bonn <jonas@southpole.se>
Signed-off-by: David S. Miller <davem@davemloft.net>