ACPI Battery: Added capacity

Added Capacity field, which is a pre-defined power_supply
property. Calculating capacity using current charge/energy and
full charge/energy.
Some user mode implementations are relying on capacity field to
show battery strength and power down decision.

Signed-off-by: srinivas pandruvada <srinivas.pandruvada@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>