pinctrl: rockchip: add separate type for rk3288

An upcoming pinctrl function of the rk3288 differs again from everything else,
so we'll need a separate type for it.

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