pinctrl: rockchip: support unrouted iomuxes per bank

On the upcoming RK3288 SoC contain some unrouted pins in their banks. So while
for example pin8 of bank5 stays pin8 with all its settings (register offset etc),
pins 0 to 7 are not routed outside the SoC at all.
Therefore add a flag to mark these unrouted iomuxes to prevent people from using
them.

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