power: supply: olpc_battery: Have the framework register sysfs files for us

The power framework gained ability to register groups of sysfs
attributes in commit cef8fe6a

 ("power: supply: core: add support for
custom sysfs attributes").

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Suggested-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>