ideapad: let camera power control entry under platform driver

The entry was at /sys/devices/LNXSYSTM:00/../VPC2004:00/camera_power
move to /sys/devices/platform/ideapad/camera_power

Add document about usage of ideapad node in sysfs.

Signed-off-by: Ike Panhc <ike.pan@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>