power: supply: bq24190_charger: add extcon support for USB OTG

Add extcon support so that we can notify USB drivers of cable state
changes. Based on work from Jonathan Marek. USB OTG was tested on a LG
Nexus 5 (hammerhead) phone.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>