pinctrl: rockchip: set is_generic in pinconf_ops

The rockchip pinctrl driver implements the generic pinconfig, therefore
also state this, so that the default pinconf dump functions work.

Signed-off-by: Heiko Stübner <heiko@sntech.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>