of-graph: export symbol of_phandle_iterator_init/next

of_for_each_phandle() uses of_phandle_iterator_init/next
but it isn't exported. So kernel module complile will say

ERROR: "of_phandle_iterator_init" [xxx.ko] undefined!
ERROR: "of_phandle_iterator_next" [xxx.ko] undefined!

This patch solved this issue

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>