pinctrl: rockchip: enable iomuxes from pmu space

The upcoming rk3288 moves some iomux settings to the pmu register space.
Therefore add a flag for this and adapt the mux functions accordingly.

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