pinctrl: rockchip: implement PIN_CONFIG_OUTPUT handling

In some cases it is nice to be able to simply control a gpio output
via the PIN_CONFIG_OUTPUT option without having a driver control it.
Thus add support for it to the rockchip pinctrl driver.

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