extcon: intel-cht-wc: Add support for registering a power_supply class-device

The bq25890 used on the Yogabook YB1-X90 / -X91 models relies on
the extcon-driver's BC-1.2 charger detection, and the bq25890 driver
expect this info to be  available through a parent power_supply
class-device which models the detected charger (idem to how the Type-C
TCPM code registers a power_supply classdev for the connected charger).

Add support for registering the power_supply class-device expected
by this setup.

Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>