arm: ls1021a: limit debug eth phy speed to 100Mbps

arm: ls1021a: limit debug eth phy speed to 100Mbps

Beside that mounted rgmii debug phy is 1000Mbps capable, the debug link
between the piggy board and the phy is 100Mbps only.
This leads to longer link establishment time when working in debug mode,
as phy tries to autoneg 1000Mbps.

This patch fixes the speed to 100Mbps and allows smother link establishment
time for the debug interface.

Signed-off-by: Aleksandar Gerasimovski <aleksandar.gerasimovski@hitachienergy.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>