EspressoBin does not use SerDes for SGMII (2.5G ethernet) but rather for SATA. Therefore Topaz switch is not connected via SerDes/SGMII but via RGMII which is limited to 1G.
Armada 3720 has 3 SerDeses and on EspressoBin one is configured in PCIe mode, another in USB3.0 and another in SATA. So there was no free SerDes for SGMII 2.5G. As Armada 3720 has also extra RGMII and authors of EspressoBin wanted to have PCIe, USB3.0 and SATA ports, the only option was to connect ethernet via extra RGMII (1Gbps).
So enabling 2.5Gbps ethernet on EspressoBin is not possible.