of: Move phandle walking to of_phandle_iterator_next()

Move the code to walk over the phandles out of the loop in
__of_parse_phandle_with_args() to a separate function that
just works with the iterator handle: of_phandle_iterator_next().

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Rob Herring <robh@kernel.org>