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>

configs/imx8mp_evk_defconfig

@@ -74,6 +74,7 @@ CONFIG_SYS_MMC_ENV_DEV=1
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
 CONFIG_USE_ETHPRIME=y
 CONFIG_ETHPRIME="eth1"
+CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_SPL_DM=y
 CONFIG_CLK_COMPOSITE_CCF=y
 CONFIG_CLK_IMX8MP=y