pinctrl: rockchip: return a complete config in pinconf_get

Till now pinconf_get only set the argument value into the config parameter
effectively removing the actual config param value. As other pinctrl drivers
do, it might be nicer to keep the config param intact.
Therefore construct a real pinconfig value from param and arg in pinconf_get

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