PM / OPP: opp-microvolt is not optional if regulators are set

If dev_pm_opp_set_regulators() is called for a device and its regulators
are set in the OPP core, the OPP nodes for the device must contain the
"opp-microvolt" property, otherwise there is something wrong and we
better error out.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>