[CPUFREQ] Documentation: ABI: /sys/devices/system/cpu/cpu#/cpufreq/

This is a complex interface and is already described in
Documentation/cpu-freq/, especially in the user-guide.txt file.

No need to copy/paste all that information. Let's just alert the reader
to the presence of the user-guide.

Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Dave Jones <davej@redhat.com>