cpufreq: cpufreq-dt: disable unsupported OPPs

If the regulator connected to the CPU voltage plane doesn't
support an OPP specified voltage with the acceptable tolerance
it's better to just disable the OPP instead of constantly
failing the voltage scaling later on.

Includes a fix to move initialization of opp_freq outside
the loop to avoid an endless loop from Geert Uytterhoeven.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>