PM / Domain: Add support to parse domain's OPP table

The generic power domains can have an OPP table for themselves now, and
phandle of their OPP nodes can be used by the devices powered by the
domain. In order for the OPP core to translate requirements between the
devices and their power domains, both need to have an OPP table in
kernel.

Parse the OPP table for power domains
if they have their
set_performance_state() callback set.

With this patch, an OPP table would be created for the genpd in kernel
based on the OPP table present in DT, if the genpd have its
set_performance_state() callback set.

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