mfd: cros_ec: Add convenience struct to define autodetectable CrOS EC subdevices

The CrOS EC is gaining lots of subdevices that are autodetectable by
sending the EC_FEATURE_GET_CMD, it takes fair amount of boiler plate
code to add those devices. So, add a struct that can be used to quickly
add new subdevices without having to duplicate code.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Tested-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>