platform/x86: system76_acpi: Add battery charging thresholds

System76 laptops running open source EC firmware support configuring
charging thresholds through ACPI methods. Expose this functionality
through the standard sysfs entries charge_control_{start,end}_threshold.

Signed-off-by: Tim Crawford <tcrawford@system76.com>
Link: https://lore.kernel.org/r/20211006202202.7479-4-tcrawford@system76.com


Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>