[ARM] DNS323: Read MAC address from flash

Based on similar code from the tsx09 series of machines, just rips the MAC
address out of flash and stuffs it into the NIC. Tested on a DNS323 rev B1.
It's possible (though unlikely) that an A1 will have the MAC in a different
location in flash.

Signed-off-by: Matt Palmer <mpalmer@hezmatt.org>