arm64: dts: imx8mp: Add dsp node

i.MX8 MPlus SoC integrates Cadence HIFI4 DSP. This core runs either a
custom firmware or the open source SOF firmware [1]

DSP device is handled by SOF OF driver found in
sound/soc/sof/sof-of-dev.c

Notice that the DSP node makes use of:
	- dsp_reserved, a reserved memory region for various Audio
	  resources (e.g firmware loading, audio buffers, etc).
	- Messaging Unit (mu2) for passing notifications betweem ARM
	  core and DSP.

[1] https://thesofproject.github.io/latest/platforms/index.html
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>