net: macb: add gigabit implementation for fixed-link

net: macb: add gigabit implementation for fixed-link

A fixed gigabit link on a non-gigabit controller is only rejected
during PHY init (even though there is no PHY to init), because, on
device-tree parsing, the controller is not probed, and it is still
unknown whether it is gigabit-capable.

This code was only tested on emulator with a full-duplex RGMII
interface, but is expected to work in GMII or half-duplex as well.

Signed-off-by: Christian DREHER <christian.dreher@nanoxplore.com>