net: ethernet: nb8800: support fixed-link DT node

Under some circumstances, e.g. when connecting to a switch, the ethernet
port will not be connected to a PHY. In that case a "fixed-link" DT node
can be used to replace it.

https://stackoverflow.com/questions/31046172/device-tree-for-phy-less-connection-to-a-dsa-switch

This patch adds support for the "fixed-link" node to the nb8800 driver.

Signed-off-by: Sebastian Frias <sf84@laposte.net>
Acked-by: Mans Rullgard <mans@mansr.com>
Cc: Mason <slash.tmp@free.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>