arm64: dts: ls1028a: move DSA CPU port property to the common SoC dtsi

Since the CPU port 4 of the switch is hardwired inside the SoC to go to
the enetc port 2, this shouldn't be something that the board files need
to set (but whether that CPU port is used or not is another discussion).

So move the DSA "ethernet" property to the common dtsi.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Michael Walle <michael@walle.cc>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>