microblaze: Enable random mac generation

In case that mac address is not found it is generated randomly.
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>

configs/microblaze-generic_defconfig

@@ -42,6 +42,7 @@ CONFIG_CMD_JFFS2=y
 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