OPP updates for 6.2

- Several DT fixes and code reorganization around opp-microvolt-<named>
  DT property (Viresh Kumar).

- Allow any of opp-microvolt, opp-microamp, or opp-microwatt properties
  to be present without the others present (James Calligeros).

- Fix clock-latency-ns prop in DT example (Serge Semin).
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