The information you linked indicating there is 2.5Gbps mode is always only for the chip (Armada 3700) used on the espressobin. This Marvell chip does have the 2.5Gbps interface. That is also the reason it is explained on Marvell github.
The espressobin itself has all 3 ethernet ports wired through one switch with only RGMII going in. As can be seen here: http://wiki.espressobin.net/tiki-index.php?page=Block+diagram
As only one RGMII is feeding all 3 ports I dont think 2.5Gbps will be possible on any of them.