imx8qxp_mek: Select CONFIG_NET_RANDOM_ETHADDR

Some i.MX8QXP MEK boards do not have MAC address stored
and hang during eth driver probe:

Error: ethernet@5b040000 address not set.

(Board hangs)

To fix this problem select CONFIG_NET_RANDOM_ETHADDR so that
a valid MAC address can be used in such case.

Signed-off-by: Fabio Estevam <festevam@gmail.com>