Limit cpufv input to acceptables values.
Add an available_cpufv file to show available
presets.
Change cpufv ouput format from %d to %#x, it won't
break compatibility with existing userspace tools, but
it provide a more human readable output.
Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Len Brown <len.brown@intel.com>