sbs-battery: add ability to get battery capacity

Battery capacity level is a standard feature of sbs battery
That can be used to tell what the remainig battery capacity is, and
can tell if the battery has not been calibrated/initialized, which makes
the capacity and charging/discharging percentages invalid.

Signed-off-by: Joshua Clayton <stillcompiling@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>