OPP: Move required opps configuration to specialized callback

The required-opps configuration is closely tied to genpd and performance
states at the moment and it is not very obvious that required-opps can
live without genpds. Though we don't support configuring required-opps
for non-genpd cases currently.

This commit aims at separating these parts, where configuring genpds
would be a special case of configuring the required-opps.

Add a specialized callback, set_required_opps(), to the opp table and
set it to different callbacks accordingly.

This shouldn't result in any functional changes for now.

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