zynq: enable CONFIG_NET_RANDOM_ETHADDR

We have to set a MAC address to use network.
Otherwise, the tftpboot command fails with the following message:

  Gem.e000b000 Waiting for PHY auto negotiation to complete........ done
  *** ERROR: `ethaddr' not set

Since commit 92ac52082140 ("net: Remove all references to
CONFIG_ETHADDR and friends"), we can not use CONFIG_ETHADDR.

The easiest way to set a MAC address is to enable
CONFIG_NET_RANDOM_ETHADDR.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/configs/zynq_zc706_defconfig b/configs/zynq_zc706_defconfig
index db11e6f..49efc6b 100644
--- a/configs/zynq_zc706_defconfig
+++ b/configs/zynq_zc706_defconfig
@@ -11,3 +11,4 @@
 # CONFIG_CMD_FLASH is not set
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_OF_EMBED=y
+CONFIG_NET_RANDOM_ETHADDR=y