arm: mvebu: clearfog gtr: add config option to select serdes0 interface

arm: mvebu: clearfog gtr: add config option to select serdes0 interface

Clearfog GTR has an assembly option for a SATA connector, CON18.
It shares the serdes with mini-pcie connector CON3.

Add new kconfig option to select betweenata and pci, defaulting to pci
as it was previously configured in board-file.

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 network link on
Clearfog GTR.

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