OPP: Make _generic_set_opp_regulator() a config_regulators() interface

In order to reuse the same code path, i.e. clk_set_rate() from
_set_opp(), migrate _generic_set_opp_regulator() to implement a
config_regulators() interface.

It is renamed to _opp_config_regulator_single() and is set as the
preferred config_regulators() interface whenever we have a single
regulator available.

Note that this also drops code responsible for restoring the
voltage/freq in case of errors. We aren't handling that properly
currently, restoring only some of the resources while leaving others out
(like bandwidth and required OPPs). It is better to drop all of it
instead of partial restoration.

Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>