Add battery charging threshold (aka ECO mode) support.

NOTE: The state of ECO mode is persistent until the next POST cycle which
reset it to previous state.
Signed-off-by: Kenneth Chan <kenneth.t.chan@gmail.com>
Link: https://lore.kernel.org/r/20200821181433.17653-9-kenneth.t.chan@gmail.comSigned-off-by: Hans de Goede <hdegoede@redhat.com>