OPP: Fix handling of multiple power domains

We seem to rely on the number of phandles specified in the
'required-opps' property to identify cases where a device is
associated with multiple power domains and hence would have
multiple virtual devices that have to be dealt with.

In cases where we do have devices with multiple power domains
but with only one of them being scalable, this logic seems to
fail.

Instead read the number of power domains from DT to identify
such cases.

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