platform/x86: asus-wmi: add support for showing charger mode

Expose a WMI method in sysfs platform for showing which connected
charger the laptop is currently using.

Signed-off-by: Luke D. Jones <luke@ljones.dev>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20230630053552.976579-2-luke@ljones.dev


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