power: supply: core: Add power_supply_prop_precharge

Battery chargers use POWER_SUPPLY_PROP_PRECHARGE_CURRENT
Clarify related item POWER_SUPPLY_PROP_CHARGE_TERM_CURRENT

Signed-off-by: Liam Breck <kernel@networkimprov.net>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>