PM / OPP: Implement dev_pm_opp_of_add_table_indexed()

The "operating-points-v2" property can contain a list of phandles now,
specifically for the power domain providers that provide multiple
domains.

Add support to parse that.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>