misc: dt-bindings: Add Qualcomm Fastrpc bindings

The FastRPC driver implements an IPC (Inter-Processor Communication)
mechanism that allows for clients to transparently make remote method
invocations across DSP and APPS boundaries. This enables developers
to offload tasks to the DSP and free up the application processor for
other tasks.

Co-developed-by: Thierry Escande <thierry.escande@linaro.org>
Signed-off-by: Thierry Escande <thierry.escande@linaro.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>