arm64: dts: lx2160a: add PCS MDIO and PCS PHY nodes

Add PCS MDIO nodes for the internal MDIO buses on the LX2160A, along
with their internal PCS PHYs, which will be used when the DPMAC is
in TYPE_PHY mode.
Also, rename the dpmac@x nodes to ethernet@x in order to be compliant
with the naming convention used by ethernet controllers.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>