MIPS: au1000_eth: Get ethernet address from platform_data

au1000_eth uses firmware calls to get a valid MAC address, and changes
it depending on platform device id.  This patch moves this logic out of
the driver into the platform device registration part, where boards with
supported chips can use whatever firmware interface they need; the default
implementation maintains compatibility with existing, YAMON-based firmware.

Tested-by: Wolfgang Grandegger <wg@denx.de>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
To: Linux-MIPS <linux-mips@linux-mips.org>
Cc: netdev@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/1481/
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>