power: supply: act8945a_charger: Add capacity level property

Add the power supply capacity level property, it corresponds to
POWER_SUPPLY_CAPACITY_LEVEL_*.

It also utilizes the precision voltage detector function module
to catch the low battery voltage.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>