gpio: Add ChromeOS EC GPIO driver

The ChromeOS embedded controller (EC) supports setting the state of
GPIOs when the system is unlocked, and getting the state of GPIOs in all
cases. The GPIOs are on the EC itself, so the EC acts similar to a GPIO
expander. Add a driver to get and set the GPIOs on the EC through the
host command interface.

Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>