pinctrl: rockchip: let pmu registers be supplied by a syscon

Currently the pmu registers containing pin pull settings on the rk3188 are mapped
locally when bank0 is instantiated. Add an alternative that can resolve the pmu
from a syscon phandle.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Max Schwarz <max.schwarz@online.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>