opp: Create _of_add_table_indexed() to reduce code duplication

The implementation of dev_pm_opp_of_add_table() and
dev_pm_opp_of_add_table_indexed() are almost identical. Create
_of_add_table_indexed() to reduce code redundancy.

Also remove the duplication of the doc style comments by referring to
dev_pm_opp_of_add_table() from dev_pm_opp_of_add_table_indexed().

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Dmitry Osipenko <digetx@gmail.com>