olpc-battery: Add VOLTAGE_MAX_DESIGN property

upowerd wants to compute the energy in the battery by looking at this
property.  If it's not present then it falls back on using the reported
voltage of the battery at time upowerd loads.  That's close but also
means that every time you boot you get a slightly different energy
capacity.

Adding the VOLTAGE_MAX_DESIGN property allows upowerd to compute the
same energy every time.

Signed-off-by: Richard A. Smith <richard@laptop.org>
Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>