microblaze: Enable random mac generation

In case that mac address is not found it is generated randomly.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/configs/microblaze-generic_defconfig b/configs/microblaze-generic_defconfig
index 1857562..7d2859c 100644
--- a/configs/microblaze-generic_defconfig
+++ b/configs/microblaze-generic_defconfig
@@ -42,6 +42,7 @@
 CONFIG_SPL_OF_CONTROL=y
 CONFIG_OF_EMBED=y
 CONFIG_DEFAULT_DEVICE_TREE="microblaze-generic"
+CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_NETCONSOLE=y
 CONFIG_SPL_DM=y
 CONFIG_XILINX_GPIO=y