mfd: cros_ec: Add char dev and virtual dev pointers

The ChromeOS Embedded Controller has to be accessed by applications.
A virtual character device is used as an interface with user-space.

Extend the struct cros_ec_device with the fields needed by the driver
of this virtual character device.

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>