dt-bindings: Add snapshot of regulator cooling device devicetree binding

Add snapshot of regulator cooling device devicetree binding to the
devicetree project. This snapshot is taken as of 'commit
<0a274ea6e54e5> ("dt-bindings: Add devicetree bindings to devicetree
project)' of the devicetree-4.19 project.

Change-Id: If4c415e6e78920fb070b16c89abb6178035c29ea