pinctrl: rockchip: add support for 4bit wide iomux settings

In the upcoming rk3288 SoC some iomux settings are 4bit wide instead of
the regular 2bit. Therefore add a flag to mark iomuxes as such and adapt
the mux-access as well as the offset calculation accordingly.

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