arm64: dts: fsl-ls1028a-kontron-sl28: specify in-band mode for ENETC

Since commit 71b77a7a ("enetc: Migrate to PHYLINK and PCS_LYNX") the
network port of the Kontron sl28 board is broken. After the migration to
phylink the device tree has to specify the in-band-mode property. Add
it.

Fixes: 71b77a7a

 ("enetc: Migrate to PHYLINK and PCS_LYNX")
Suggested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20201109110436.5906-1-michael@walle.cc
Signed-off-by: Jakub Kicinski <kuba@kernel.org>