Documentation: power: Initial effort to document power_supply ABI

This commit adds generic ABI information regarding power_supply
properties. This is an initial attempt to try and align the usage
of these properties between drivers. As part of this commit,
common Battery and USB related properties have been listed.

Signed-off-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>