[CPUFREQ] document the currently undocumented parts of the sysfs interface

There is a description of some of the sysfs files.  However, there are some
that are not mentioned in the documentation, so add them to the user's guide.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Cc: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Jones <davej@redhat.com>