cpufreq: docs: Add missing scaling_available_frequencies description

Add a description of the scaling_available_frequencies attribute in
sysfs to the documentation.

Signed-off-by: Raphael Gallais-Pou <rgallaispou@gmail.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://patch.msgid.link/20240701171040.369030-1-rgallaispou@gmail.com


Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>