Not like other Intel Ethernet controllers (e.g.: E1000), Intel Quark
SoC integrated designware Ethernet controller does not have a chipset
defined way to store/restore mac address. Enable random mac address
so that we can use Ethernet even without 'ethaddr'.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>