[POWERPC] Avoid putting cpu node twice

Call of_find_node_by_type with NULL instead of np
so the cpu node does not get put twice.
This was causing kref_put warnings.

Signed-off-by: John Rigby <jrigby@freescale.com>
Acked-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>