Without this, building drivers/serial/of_serial.c as a module fails.

WARNING: ".of_find_property" [drivers/serial/of_serial.ko] undefined!
Signed-off-by: Dave Jones <davej@redhat.com>
Acked-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>