ath79: fix nanobeam ac ethernet interface

ath79: fix nanobeam ac ethernet interface

In 4.14 the delays were not cleared, so setting "rgmii" as phy-mode
did not affect delays set by the bootloader. With 5.4 kernel the
situation changed and the ethernet interface stopped working.

"rgmii" requires rx and tx delays depending on the hardware circuit
and wiring. The mac or the phy can add these delays.
- "rgmii":  delays are controlled by the mac
- "rgmii-id": delays are controlled by the phy
More Information in Linux Kernel Tree:
Documentation/devicetree/bindings/net/ethernet-controller.yaml

"rgmii" should be the preferred mode, which allows the mac layer to
turn off the dealys completely if they are not needed. However, the
delays are not set correctly, which causes the ethernet interface
to be broken. Just taking the ethernetpart from the litebeam ac gen2
will fix the issue.

Explained-by: David Bauer <mail@david-bauer.net>
Signed-off-by: Nick Hainke <vincent@systemli.org>