powerpc: make it compile for multithread change

arch/powerpc/kernel/of_platform.c:479: error: unknown field `multithread_probe' specified in initializer

Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>