OPP: Allow OPP table to be used for power-domains

Power-domains can also have their active states and this patch enhances
the OPP binding to define those. The power domains can use the OPP
bindings as is, with one additional change to Allow
"operating-points-v2" property to contain multiple phandles for power
domain providers providing multiple domains.

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