imx8mp_evk: Select CONFIG_NET_RANDOM_ETHADDR

On an early revision of the imx8mp-evk that I have access to,
the MAC addresses fuses are not programmed, causing failure to bring
the Ethernet interfaces.

Fix this problema by selecting CONFIG_NET_RANDOM_ETHADDR so that
random MAC addresses are assigned and the Ethernet ports become
functional out of the box.

Signed-off-by: Fabio Estevam <festevam@denx.de>