Revert "power: remove POWER_SUPPLY_PROP_CAPACITY_LEVEL"

This reverts commit 8efe4440 and
4cbc76ea.

Richard@laptop.org was apparently using CAPACITY_LEVEL for debugging
battery/EC problems, and was upset that it was removed.  This readds it.

Conflicts:

	Documentation/power_supply_class.txt

Signed-off-by: Andres Salomon <dilinger@collabora.co.uk>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>