power: supply: bq25890: Clean up POWER_SUPPLY_PROP_CONSTANT_CHARGE_VOLTAGE

Clean up misuse of POWER_SUPPLY_PROP_VOLTAGE,
POWER_SUPPLY_PROP_CONSTANT_CHARGE_VOLTAGE_MAX
and POWER_SUPPLY_PROP_CONSTANT_CHARGE_VOLTAGE
and document what exactly each value means.

The POWER_SUPPLY_PROP_CONSTANT_CHARGE_VOLTAGE content is newly read
back from hardware, while POWER_SUPPLY_PROP_CONSTANT_CHARGE_VOLTAGE_MAX
is reported as the maximum value set in DT.

The POWER_SUPPLY_PROP_VOLTAGE is newly used to report immediate value
of battery voltage V_BAT, which is what this property was intended to
report and which has been thus far misused to report the charger chip
output voltage V_SYS.

The V_SYS is no longer reported as there is currently no suitable
property to report V_SYS. V_SYS reporting will be reinstated in
subsequent patch.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>