Both RDU2 and RDU3 share the same MAC address configuration
mechanism. Reflect that by moving that code into zii-common and
changing it to be executed on both platforms.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>