PM / OPP: Add missing doc comments

Few doc-style comments were missing, add them. Rearrange another one to
match the sequence within the structure.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>