power: battery: add sysfs to configure max percentage

Some users might want to extend their battery lifes in the
long term by not charging to the very max. This little patch
enables this feature, the default max value is 100 (%)
which can be controlled via;
/sys/kernel/battery_control/max_battery_level

e.g. echo 95 > /sys/kernel/battery_control/max_battery_level

Signed-off-by: Blechd0se <alex.christ@hotmail.de>