arm64: dts: imx8mp: Configure dsp node for rproc usage

DSP can be used with various frameworks (e.g audio firmware, rproc).
Currently 'dsp' configuration is intended for audio firmware but it
doesn't work well with board level DTs (e.g imx8mp-evk) because
board level DT enables audio related IPs (e.g SAI) while audio firmware
needs this IPs disabled (because firmware will configure them).

So, configure 'dsp' node to be used with rproc. This way users will be
able to use board DT to use the DSP as long as they don't clash with
Audio IP configurations.

More comples usage of 'dsp' node (e.g by audio firmware) will need to
create a separate dts file (or an overlay).

This change follows the approach taken for other i.MX8 boards
in commit 391a319c ("arm64: dts: imx8-ss-audio: configure dsp node
for rproc usage")

Reviewed-by: Iuliana Prodan <iuliana.prodan@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>