arm64: dts: ls1028: sl28: fix networking for variant 2

The PHY configuration for the variant 2 is still missing the flag for
in-band signalling between PHY and MAC. Both sides - MAC and PHY - have
to match the setting. For now, Linux only supports setting the MAC side
and thus it has to match the setting the bootloader is configuring.
Enable in-band signalling to make ethernet work.

Fixes: ab43f030 ("arm64: dts: ls1028a: sl28: add support for variant 2")
Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>