MIPS: DTS: CI20: make DM9000 Ethernet controller use NVMEM to find the default MAC address

There is a unique MAC address programmed into the eFuses
of the JZ4780 chip in the CI20 factory. By using this
for initializing the DM9000 Ethernet controller, every
CI20 board has an individual - but stable - MAC address
and DHCP can assign stable IP addresses.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>