pinctrl: actions: Add Actions S900 pinctrl driver

Add pinctrl driver for Actions Semi S900 SoC. The driver supports
pinctrl, pinmux and pinconf functionalities through a range of registers
common to both gpio driver and pinctrl driver.

Pinmux functionality is available only for the pin groups while the
pinconf functionality is available for both pin groups and individual
pins.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>