misc: fastrpc: add secure domain support

ADSP/MDSP/SDSP are by default secured, which means it can only be loaded
with a Signed process.
Where as CDSP can be either be secured/unsecured. non-secured Compute DSP
would allow users to load unsigned process and run hexagon instructions,
but blocking access to secured hardware within the DSP. Where as signed
process with secure CDSP would be allowed to access all the dsp resources.

This patch adds basic code to create device nodes as per device tree property.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>