cpufreq_dt: Improve multiple opp support

When looking for cpu with the same OPP starts from the root /cpus node
so each instance of cpufreq_dt will now each cpu with the same operating
point.
Also test that the node we are testing have the property "device_type" set
to be equal to "cpu".
While here add more debug printfs (off by defaults).

MFC after: 2 weeks