thinkpad_acpi: Add support for battery thresholds

thinkpad_acpi: Add support for battery thresholds

1) Charge start threshold
/sys/class/power_supply/BATN/charge_start_threshold

Valid values are [0, 99]. A value of 0 turns off the
start threshold wear control.

2) Charge stop threshold
/sys/class/power_supply/BATN/charge_stop_threshold

Valid values are [1, 100]. A value of 100 turns off
the stop threshold wear control. This must be
configured first.

Signed-off-by: Ognjen Galic <[email protected]>
Acked-by: Henrique de Moraes Holschuh <[email protected]>
Signed-off-by: Rafael J. Wysocki <[email protected]>