arm64: dts: fsl-ls1028a-rdb: Specify in-band mode for ENETC port 0

As part of the transition of the enetc ethernet driver from phylib
to phylink, the in-band operation mode of the SGMII interface
from enetc port 0 needs to be specified explicitly for phylink.

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>