ks8842: Add platform data for setting mac address

This patch adds platform data to the ks8842 driver.

Via the platform data a MAC address, to be used by the controller,
can be passed.

To ensure this MAC address is used, the MAC address is written
after each hardware reset.

Signed-off-by: Richard Röjfors <richard.rojfors@pelagicore.com>
Signed-off-by: David S. Miller <davem@davemloft.net>