gpio: of: add missing of_node_put() to of_gpiochip_add_pin_range()

As the comment block of of_parse_phandle_with_fixed_args() says,
the caller is responsible to call of_node_put() on the returned
node when done.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>