power: supply: add helpers for charge_behaviour sysfs

These helper functions can be used by drivers to implement their own
sysfs-attributes.
This is useful for ACPI-drivers extending the default ACPI-battery with
their own charge_behaviour attributes.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20211123232704.25394-3-linux@weissschuh.net


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