pinctrl: rockchip: precalculate iomux offsets

An upcoming SoC introduces an interesting quirk to iomux handling making the
calculation of the iomux register-offset harder. To keep the complexity down
when getting/setting the mux, precalculate the actual register offset at
probe-time.

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