board: lx2160aqds: fix ethernet-phy compatible property

The code that generates the compatible property concatenates the
ethernet phy id and clause-compatible information without
separating them with a comma, resulting into no ethernet phy driver
getting loaded by Linux kernel.
Suffix phy_id_compatible_str with comma to fix this

Signed-off-by: Florin Chiculita <florinlaurentiu.chiculita@nxp.com>
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>