net: ks8851: Support DT-provided MAC address

Allow the boot loader to specify the MAC address in the device tree
to override the EEPROM, or in case no EEPROM is present.

Cc: Ben Dooks <ben@simtec.co.uk>
Cc: Tristram Ha <tristram.ha@micrel.com>
Cc: David J. Choi <david.choi@micrel.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: David S. Miller <davem@davemloft.net>