Documentation: power: Update outdated contents in opp.rst

Three problems have been revised:
1. CONFIG_PM_OPP no longer depends on CONFIG_PM.
2. opp_find_freq_{ceil, floor} is renamed as dev_pm_opp_find_freq_{ceil,
   floor}.
3. Make it clear that the 'opp' struct is actually dev_pm_opp.

Signed-off-by: Tang Yizhou <tangyizhou@huawei.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>