mfd / platform: cros_ec: Move vbc attributes to its own driver

The entire way how cros sysfs attibutes are created is broken.
cros_ec_vbc should be its own driver and its attributes should be
associated with a vbc driver not the mfd driver. In order to retain
the path, the vbc attributes are attached to the cros_class.

The patch also adds the sysfs documentation.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>