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 92ac5208 ("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>