km/powerpc: remove unneeded defines

We use CONFIG_OF_LIBFDT and CONFIG_OF_BOARD_SETUP on all our powerpc
targets, so there is no need to check these defines within our C code.

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