eeepc-wmi: Add cpufv sysfs interface

eeepc-laptop provides a sysfs interface to read and control what it
calls cpufv.  When WMI is enabled, the ACPI interface changes slightly
and becames a write-only control with 3 valid values.

Expose cpufv again to allow for user space utils that can extended battery
life noticably and come a little closer to parity with eeepc-laptop.

Write-only is OK for most user space apps because read status was
mostly used to prevent unneeded mode changes.  Since this same check
to ignore changes to same mode also exists in the DSDT then it was
wasted ACPI call.

acpi_osi="!Windows 2009" can be used for get back eeepc-laptop's
read support of cpufv for debugging things such as behaviour
during resume.

This patch was tested with EEE PC 1005PE by monitoring powertop output while
writing values of "0", "1", and "2" and by reviewing the decompiled DSDT of
an 1201NL and comparing it to 1005PE's DSDT.

Signed-off-by: Chris Bagwell <chris@cnpbagwell.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>