arm64: dts: ls208xa: add the external MDIO nodes

Add the external MDIO device nodes found in the WRIOP global memory
region. This is needed for management of external PHYs.

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