Documentation: power: rt9471: Document exported sysfs entries

Document the settings exported by rt9471 charger driver through sysfs entries:
- sysoff_enable
- port_detect_enable

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
[update kernel version and date]
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>