board: solidrun: clearfog: fix serdes 1 / eth2 speed for clearfog gtr

Clearfog GTR connects eth2 / serdes 1 to a 2.5Gbps capable ethernet
switch port. Linux already configures a fixed-link at speed 2500 from
device-tree.

Upgrade serdes 1 rate to 3.125Gbps to support a 2.5Gbps link.

Additionally add comments documenting each serdes' function of clearfog
gtr, which are shared with clearfog pro.

Signed-off-by: Josua Mayer <josua@solid-run.com>