Targeting mobile and consumer hardware where there may not be dedicated VP9 hardware, Ittiam has developed an OpenCL-based encoder that will work with the likes of ARM Mali graphics hardware. Obviously doing this encoding on the GPU in software is more efficient and performant than a CPU-based software encoder. The Samsung S6 was demonstrated with this OpenCL-based encoder for efficient VP9 usage.
Sadly this OpenCL-based VP9 video encoder isn't open-source/free software but Ittiam appears to be licensing it to various OEMs. More details via this press release. Ittiam had previously already done an OpenCL-based VP9 decoder.
Hopefully someone in the open-source community will take in time to writing a OpenCL-based VP9 implementation, but then again the open-source OpenCL Linux driver stacks still leave a lot to be desired.