net-next: mediatek: properly handle RGMII modes

If an external Gigabit PHY is connected to either of the MACs we need to
be able to tell the PHY to use a delay. Not doing so will result in heavy
packet loss and/or data corruption when using PHYs such as the IC+ IP1001.
We tell the PHY which MII delay mode to use via the devictree.

The ethernet driver needs to be adapted to handle all 3 rgmii-*id modes
in the same way as normal rgmii when setting up the MAC.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>