km/powerpc: move open firmware defines to km-powerpc.h

We use the same settings for open firmware defines on all our powerpc
targets, so move them from the CPU specific headers to the common
powerpc header.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>