of: unify of_count_phandle_with_args() arguments with !CONFIG_OF

Unify the declaration of of_count_phandle_with_args() between enabled
and disabled OF by making constifying pointed device_node.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>