OPP: Migrate set-regulators API to use set-config helpers

OPP: Migrate set-regulators API to use set-config helpers

Now that we have a central API to handle all OPP table configurations,
migrate the set-regulators family of helpers to use the new
infrastructure.

The return type and parameter to the APIs change a bit due to this,
update the current users as well in the same commit in order to avoid
breaking builds.

Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>