change to GMAC easier on A20 SoCs.
On A10 only the EMAC controller is available (fast ethernet), but on A20
there is also GMAC a high (or better) performant controller (gigabit
ethernet).
On A20 the both controllers uses the same pins to talk to the ethernet PHY
(MII or RGMII) and they can be selected by the GPIO pin mux.
There is work in progress to bring in GMAC support.