zynqmp: 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: NMichal Simek <michal.simek@xilinx.com>

configs/xilinx_zynqmp_ep_defconfig

@@ -20,4 +20,5 @@ CONFIG_CMD_DHCP=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_TIME=y
 CONFIG_CMD_TIMER=y
+CONFIG_NET_RANDOM_ETHADDR=y
 # CONFIG_REGEX is not set