mfd: cros_ec: Add API for Touchpad support

Add API to control touchpad presented by Embedded Controller.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Acked-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Acked-by: Benson Leung <bleung@chromium.org>
Reviewed-by: Fabien Lahoudere <fabien.lahoudere@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>