ACPI / battery: introduce support for POWER_SUPPLY_PROP_CAPACITY_LEVEL

ACPI battery device receives notifications when
1. the remaining battery capacity becomes critical low
2. the trip point set by the _BTP (Design capacity of Warning by default)
   is reached or crossed.

So it is able to support POWER_SUPPLY_PROP_CAPACITY_LEVEL to report
        POWER_SUPPLY_CAPACITY_LEVEL_CRITICAL,
        POWER_SUPPLY_CAPACITY_LEVEL_LOW,
        POWER_SUPPLY_CAPACITY_LEVEL_NORMAL,
        POWER_SUPPLY_CAPACITY_LEVEL_FULL,
capacity levels to power supply core and user space.

Introduce support for POWER_SUPPLY_PROP_CAPACITY_LEVEL in this patch.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>