dt-bindings: thermal: Snapshot of lmh cpu VDD driver document from msm-4.19

Add a snapshot of limits cpu voltage cooling device driver document
from msm-4.19. This is snapshot as of 'commit <683e0411b258>
("dt-bindings: thermal: Add limits CPU voltage cooling device")'.

Change-Id: I2de821ec34d89e0c9d1702ebb322267f4c8ea943