cpufreq: interactive: Add documentation for new sysfs nodes

Add documentation to describe sysfs nodes that are currently not
documented.

Change-Id: Ib67d401afbb738e1ab79f73f34fab13922c3d98e
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>