dt-bindings: net-next: Add DT bindings documentation for Allwinner V3s EMAC

Allwinner V3s SoC has a Ethernet MAC like the one in Allwinner H3, but
have no external MII capability. That means that it can only use the
EPHY and cannot do Gbps transmission.

Add binding for it.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: David S. Miller <davem@davemloft.net>