Add dsp backend in ggml.h

[jeffzhou2000]

could a dsp backend be added in this file for fix an opening issue #771?

GGML_BACKEND_TYPE_DSP = 30,

https://github.com/ggerganov/ggml/blob/master/include/ggml/ggml.h#L385

[slaren]

The aim is to remove ggml_tensor::backend, so we do not want to add to it. New backends should use the buffer type to identify the location of a tensor.

[jeffzhou2000]

thanks for your comment. I see now.