net: systemport: allow changing MAC address

Hook a ndo_set_mac_address callback, update the internal Ethernet MAC in
the netdevice structure, and finally write that address down to the
UniMAC registers. If the interface is down, and most likely clock gated,
we do not update the registers but just the local copy, such that next
ndo_open() call will effectively write down the address.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>