eeepc-wmi: add cpufv sysfs documentation

Based on cpufv text from sysfs-platform-eeepc-laptop that
has almost same behavior.

Signed-off-by: Chris Bagwell <chris@cnpbagwell.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Acked-by: Corentin Chary <corentincj@iksaif.net>