gpio/rockchip: fix refcount leak in rockchip_gpiolib_register()

The node returned by of_get_parent() with refcount incremented,
of_node_put() needs be called when finish using it. So add it in the
end of of_pinctrl_get().

Fixes: 936ee267 ("gpio/rockchip: add driver for rockchip gpio")
Signed-off-by: Wang Yufen <wangyufen@huawei.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>