dt-bindings: Add snapshot of RPM SMD devicetree binding

Add snapshot of RPM SMD devicetree binding to the devicetree project.
This snapshot is taken as of 'commit <f7835e76aab01> ("drivers:
thermal: Add support for RPM SMD cooling device")' of the msm-4.19
kernel project.

Change-Id: I66e9017e88d28b1c1407cb3d3673ad41be1c280a