powerpc/82xx: merge mgcoge.h and mgcoge3ne.h into km82xx.h

Since mgcoge and mgcoge3ne are the only km82xx boards, there is no need
to keep them as separate .h config files.
Therefore, make mgcoge3ne.h and mgcoge.h converge into a single km82xx.h
file.

Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>