power_supply: wilco_ec: Add charging config driver

Add a driver to control the charging algorithm used on Wilco
devices. See Documentation/ABI/testing/sysfs-class-power-wilco
for the userspace interface and other info.

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