ARM: dts: imx: m53menlo: Convert to DM_ETH

Convert the board to DM_ETH instead of legacy networking. This requires
a minor addition to the DT to satisfy the requirement for specifying a
PHY node. No functional change from board user perspective.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP i.MX U-Boot Team <uboot-imx@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>