amd-xgbe: add 2.5GbE support to 10G BaseT mode

Add support to the driver to fully recognize and enable 2.5GbE speed in
10GBaseT mode.

Acked-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Signed-off-by: Raju Rangoju <Raju.Rangoju@amd.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>