mfd: cros_ec: Instantiate ChromeOS EC character device

The ChromeOS EC character device is an user-space interface to
allow applications to access the Embedded Controller.

Add a cell for this device so it's spawned from the mfd driver.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Acked-by: Lee Jones <lee.jones@linaro.org>
Tested-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Olof Johansson <olof@lixom.net>