Let the PHY generate the RX and TX delay on the Odroid-C1 and MXIII
Plus.

Previously we did not know that these boards used an RX delay. We
assumed that setting the TX delay on the MAC side It turns out that
these boards also require an RX delay of 2ns (verified on Odroid-C1,
but the u-boot code uses the same setup on both boards). Ethernet only
worked because u-boot added this RX delay on the MAC side.

The 4ns TX delay was also wrong and the result of using an unsupported
RGMII TX clock divider setting. This has been fixed in the driver with
commit bd6f4854 ("net: stmmac: dwmac-meson8b: Fix the RGMII TX
delay on Meson8b/8m2 SoCs").

Switch to phy-mode "rgmii-id" to let the PHY side handle all the delays,
(as recommended by the Ethernet maintainers anyways) to correctly
describe the need for a 2ns RX as well as 2ns TX delay on these boards.
This fixes the Ethernet performance on Odroid-C1 where there was a huge
amount of packet loss when transmitting data due to the incorrect TX
delay.
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20200512215148.540322-3-martin.blumenstingl@googlemail.com