power: supply: cpcap-battery: Fix handling of lowered charger voltage

With cpcap-charger now using 4.2V instead of 4.35V, we never reach
POWER_SUPPLY_CAPACITY_LEVEL_FULL unless we handle the lowered charge
voltage.

Let's do this by implementing POWER_SUPPLY_PROP_CONSTANT_CHARGE_VOLTAGE,
and assume anything at that level or higher is a full battery.

Let's also make it configurable for users who may still want to
reconfigure it, and notify the charger if supported by the charger.

Cc: Merlijn Wajer <merlijn@wizzup.org>
Cc: Pavel Machek <pavel@ucw.cz>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>