powerpc: Guard print_device_node_tree() with #if 0

Currently print_device_node_tree() isn't called but it can be useful for
debugging.  Leave the function there but hide it behind '#if 0' to save
it being rewritten.  If you want to call it you're already editing this
file anyway. ;P

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>