ppc: add support for ppc based board mgcoge2ne

The mgcoge2 board from keymile deploys two different processors.
An ARM based Kirkwood for the "unit" part of the SW and a PPC for
the "ne" part of the SW. Therefore in Linux and U-Boot the names
for the board are mgcoge2un and mgcoge2ne. This patch adds the
mgcoge2ne part of the board. The ppc part of mgboge2 is quite
similar to mgcoge, therefore a generic header km82xx-common.h
was introduced to collect all similiarities. Currently the only
difference is that mgcoge2ne has a 64 MB numonyx NOR flash with
a single die. The mgcoge has a dual die flash 2*32MB from spansion.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <wd@denx.de>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>