gpio: em: Add pinctrl support

Register the GPIO pin range, and request and free GPIO pins using the
pinctrl API. The pctl_name platform data member should be used by
platform devices to point out which pinctrl device to use.

Follows same style as "dc3465a9 gpio-rcar: Add pinctrl support",
by Laurent Pinchart, thanks to him.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>