Support new charge_control_*_threshold

This is now documented as the standard Linux kernel API for
charging thresholds:
https://github.com/torvalds/linux/blob/master/Documentation/ABI/testing/sysfs-class-power

This was added to the `thinkpad_acpi` driver in
https://github.com/torvalds/linux/commit/e33929537b76486d2ed576a0d9ce3ebff51bf851



And keep backward compatible code path for now.

Co-authored-by: Ian Douglas Scott <idscott@system76.com>