platform/chrome: wilco_ec: Add Boot on AC support

Boot on AC is a policy which makes the device boot from S5 when AC
power is connected. This is useful for users who want to run their
device headless or with a dock.

Signed-off-by: Nick Crews <ncrews@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>