pinctrl: rockchip: add drive-strength control for rk3288

The rk3288 is the first Rockchip soc handling the drive strength on a per-pin
basis, while the older ones can set the drive-strength only for specific
pin-groups. Therefore limit setting the drive-strength to this soc for now.

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