board: phytec: phycore_imx8mp: Add fec support

Enable support for the fec ethernet on phyCORE-i.MX8MP.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>