powerpc/prom: Use of_get_next_parent() in of_get_ibm_chip_id()

Use of_get_next_parent() to simplifiy the logic in of_get_ibm_chip_id().

Original-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>