platform/chrome: wilco_ec: Add charging config driver

Add a device to control the charging algorithm used on Wilco devices,
which will be picked up by the drivers/power/supply/wilco-charger.c
driver. See Documentation/ABI/testing/sysfs-class-power-wilco for the
userspace interface and other info.

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