pinctrl: add pinctrl driver for Rockchip SoCs

This driver adds support the Cortex-A9 based SoCs from Rockchip,
so at least the RK2928, RK3066 (a and b) and RK3188.
Earlier Rockchip SoCs seem to use similar mechanics for gpio
handling so should be supportable with relative small changes.
Pull handling on the rk3188 is currently a stub, due to it being
a bit different to the earlier SoCs.

Pinmuxing as well as gpio (and interrupt-) handling tested on
a rk3066a based machine.

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