MIPS: Alchemy: Rewrite ethernet platform setup

Rewrite ethernet setup to use runtime cpu detection, and also clean up
the ethernet base address mess as far as possible.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
To: Linux-MIPS <linux-mips@linux-mips.org>
Cc: Florian Fainelli <florian@openwrt.org>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Patchwork: https://patchwork.linux-mips.org/patch/2353/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org