MIPS: Alchemy: Fix Au1100 ethernet build failure

Don't define platform info for second mac on au1100 (which only has a
single mac).

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
To: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: http://patchwork.linux-mips.org/patch/1004/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>