[POWERPC] cell: safer of_has_vicinity routine

This patch changes the way we check for the existence of
vicinity property in spe device nodes.

The new implementation does not depend on having an initialized
cbe_spu_info[0].spus, and checks for presence of vicinity in all
nodes, not only in the first one.

Signed-off-by: Andre Detsch <adetsch@br.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>